The word "substantial" can mean that something is not ideal but instead is real and tangible. (Something "tangible" can be touched. This is related to the word "tactile.") In contrast to something substantial, the word "intangible"—not touchable—is an excellent antonym.

When something is "opaque," that thing prevents light from passing through it. For example, a house wall is opaque to the light in a given room, preventing it from passing into the next room. In contrast to this, the windows in the room are "translucent," meaning that they allow light through them. This word comes from the combination of the prefix "trans-," meaning across, and "lucent," which comes from light. Translucent surfaces literally allow light to shine through or across them.

"Placid" is an adjective that can mean calm; not easily excited when talking about a living organism. It can also mean calm when describing water. When taking a test, if there is more than one meaning for a word, it is best to look at all of the possible answers and eliminate the ones that don't fit any definition.

Using this strategy, we can eliminate "tranquil," because it means peaceful. This is a synonym of "placid," not an antonym.

"Perplexed" can also be eliminated because it means confused. This is not the opposite of "calm."

"Innocent" means harmless and free of guilt. Again, this is not the opposite of calm.

This leaves us with two possibilities: "excitable" and "aggravating." "Excitable" is an adjective meaning easily excited. "Aggravating" is an adjective meaning irritating. "Excitable" is the best antonym for the first definition of "placid."