Galactose and glucose are both 6-carbon monosaccharides. They are classified as structural isomers. This means that they have the same molecular formula, but different structural orientations. The molecular formula of a generic monosaccharide with  carbons is ; therefore, the molecular formula for both glucose and galactose is . They have the same number of hydrogen and oxygen atoms.

The chemical structures of glucose and galactose differ in the C4 atom. The hydroxyl group is oriented differently at this position, altering the stereochemistry at C4. All other carbon atoms in glucose and galactose have the same stereochemistry. This means that glucose and galactose are a special type of structural isomer called epimers.

Sucrose is a disaccharide that is made up of a glucose and a fructose molecule, bound by a glycosidic linkage. A disaccahridase, called sucrase, breaks down sucrose molecules into their component monosaccharides (glucose and fructose), which can then by absorbed by the enterocytes in the small intestine.

Fructose is a monosaccharide that can be directly absorbed by enterocytes. Starch is a complex carbohydrate (polysaccharide) with many glucose molecules attached via glycosidic bonds. 

Just like starch, glycogen and cellulose are complex carbohydrates that contain glucose molecules joined via glycosidic bonds. Both carbohydrates contain (1,4) glycosidic bonds. This means that the bond occurs between the 1-carbon of one glucose and the 4-carbon of the other glucose. Glycogen contains -(1,4) glycosidic bonds whereas cellulose contains -(1,4) bonds.

In addition, glycogen molecules contain -(1,6 ) glycosidic bonds. These bonds are typically found in branch points along the main chain of glucose molecules. Cellulose molecules do not have branching and, therefore, do not have -(1,6) glycosidic bonds.

As mentioned, glycogen contains  bonds. Humans have enzymes capable of digesting complex carbohydrates with  linkages; therefore, only glycogen and starch (which also has  linkages) can be digested by humans. Amylin and amylopectin are forms of starch, not cellulose. 

The question states that the molecule has the same structure as glucose, except at one stereogenic center. Recall that epimers are structural isomers that differ at one stereogenic center; therefore, glucose and the compound are epimers. The epimers of glucose include galactose and mannose. All three molecules have a molecular formula of .

Recall that aldoses are carbohydrates that have an aldehyde group at one of the carbons. Glucose and all of its epimers have an aldehyde group at the first carbon; therefore, this compound is an aldose.

Fructose is a ketose that has a ketone group in the second carbon. Recall that aldehydes and ketones have carbonyl groups; therefore, they are not considered stereogenic centers (to be a stereogenic center you need to have four distinct functional groups on a carbon). Since fructose differs from glucose at a non-stereogenic center, this molecule cannot be fructose. Note that fructose also has a different molecular formula (). 

Only 1% of all sugars that have five or more carbons are found in an open chain, thus any sugar that has five or more carbons will be most likely found in its cyclic form. Of the four choices a triose is the only one that has less that five carbons (it has three) the others have 5 (pentose), 6 (hexose), and 7 (heptose).

Given no other information about how many carbons are in the chain, any monosaccharide with a carbonyl group on a carbon between two others and not at the terminal carbon of the chain is called a ketose. Aldoses are when the carbonyl group is at the end of the chain. Ribose and aldohexose are incorrect because 1) ribose is too specific, we do not know anything about the structure and ribose indicates a specific monosaccharide, and 2) aldohexose has an aldehyde group, not a ketone group. 

A furanose is defined as a cyclical sugar structure with a five-membered ring. By contrast, a pyranose is a cyclical sugar structure with a six-membered ring.

Three common simple sugars are: glucose, fructose, and galactose. Combining these simple sugars leads to the formation of more complex sugar molecules. Glucose and fructose make sucrose. Glucose and galactose make lactose. Two glucose molecules make maltose. 

A pyranose is a carbohydrate that includes a ring. It is not an open-chain carbohydrate. Additionally, this term is reserved for six-membered, not five-membered carbohydrate ring structures. A sugar which contains a five-membered ring as part of its cyclic structure is called a furanose.

As carbohydrates are broken down, they are oxidized to carbon dioxide . This process, cellular respiration, begins during glycolysis. Reduction is the opposite of oxidation, and would indicate a loss of oxygen. Elimination refers to the removal of a functional group, which does not occur during this process. In substitution, one functional group is replaced with another, which also does not accurately describe this process. Finally, proteolysis refers to the breakdown of proteins, not carbohydrates.