Elements within a group have the same number of valence electrons, but in increasing energy levels. Elements toward the bottom of a group have valence electrons with higher energies in larger orbitals. This results in a larger radius and a weaker attractive force between the nucleus and outer electrons. The ionization energy decreases as the electrons are more removed from the attraction of the nucleus.

When moving down a group, atomic radius increases and ionization energy decreases.

Ionization energy is the energy that an atom must absorb in order to release an electron. Metals have fairly low ionization energies, up until they develop a noble gas electron configuration. At that point, the ionization energy will spike due to the stability of the noble gas configuration. For example, sodium has a very low first ionization energy because it has only one valence electron, but a very high second ionization energy because removal of a second electron disrupts the noble gas configuration of the ion.

Based on its position on the periodic table, scandium shows that it can release three electrons before it develops a noble gas electron configuration (hence its tendency to become an ion with a charge of +3).

At this point, the atom will require a massive amount of absorbed energy in order to release a fourth electron. This means that the 4th ionization energy for scandium is the highest of the given options.

First ionization energy tends to decrease as you move downward and to the left on the periodic table. Of the given answer options, rubidium is the farthest down and to the left, so it has the lowest first ionization energy.

Ionization energy is a property of an atom that describes the lowest energy needed to remove a electron from its valence shell in the ground state.

Ionization energy increases up and to the right on the periodic table. All the elements listed are halogens located in group 7A of the periodic table. Because fluorine is located at the top of the periodic table in group 7A, it will have the highest first ionization energy. Ionization energy can be though of as the inverse of atomic radius. That is, the closer the valence shell of electrons is to the nucleus, the larger the magnitude of electric attractive force, and thus, the more energy is required to strip that electron. 

Ionization energy is a property of an atom or ion that describes the lowest energy needed to remove a electron from it in the ground state.

Ionization energy increases up and to the right on the periodic table. Because oxygen is the highest and rightmost element of those listed, it will have the highest first ionization energy. Ionization energy can be though of as the inverse of atomic radius. That is, the closer the valence shell of electrons is to the nucleus, the larger the magnitude of electric attractive force, and thus, the more energy is required to strip that electron.

Ionization energy is a property of an atom or ion that describes the lowest energy needed to remove a electron from it in the ground state.

Ionization energy increases up and to the right on the periodic table. Because fluorine is located furthest to the top right on the periodic table in group 7A, it will have the highest first ionization energy.

Ionization energy is a property of an atom or ion that describes the lowest energy needed to remove a electron from it in the ground state.

Ionization energy increases up and to the right on the periodic table. All the elements listed are either lower than or further to the left on the periodic table than chlorine, therefore chlorine has the highest ionization energy.

Ionization energy is a property of an atom or ion that describes the lowest energy needed to remove a electron from it in the ground state.

Ionization energy increases up and to the right on the periodic table. All the elements listed are chalcogens located in group 6A of the periodic table. Because oxygen is located furthest to the top right on the periodic table in group 6A, it will have the highest first ionization energy.