The prefix "under-" means beneath or below, so the word “underage” means below age, immature. To provide further help, a “novice” is a beginner and an “expert” is someone who has been doing something for a long time and is very good at it; a “vendor” is someone who sells things; a “genius” is someone who is very smart. 

“Dismount” means to get down. It is commonly used when speaking about “dismounting" from horses, as in to get off your mount. The prefix "dis-" is a negation prefix and the suffix "-mount" refers to something you sit on.

The prefix "semi-" usually means half, so we can assume that a “semicircle” is half a circle. To provide further help, a three-dimensional circle is called a “sphere;” “parallel” means running alongside, having the same slope; “slope” means slanted ground; “gradient” means slope. In addition to being terms we use in English, these are also all mathematical terms. 

The prefix "under-" means below or beneath, so the word “underdeveloped” means not grown enough, not developed enough. To provide further help, “developed” means grown; “beneath” means under. 

The suffix "-act" means to do, to act, to respond. The word “react” means respond to something. To provide further help, “writhe” means twist in pain; “egress” means exit, way out; “akin” means similar to; “variegated” means different looking, multicolored.

The prefix "over-" means completely, excessively, too much, so the word “overburdened” means carrying too much of a load, or "overloaded.” To provide further help, “overlook” means ignore; “overhead” means above; "overt" means secret.

The prefix "infra-" generally means below or under, so the word “infrastructure” means basic structure or "foundation." To provide further help, a “trial” is a test; a “variable” is something that can change; a “luminary” is a famous or successful person. 

The prefix "an-" means toward, to join, or to add onto. The word “annex,” when used as a verb, means add or join, and it is most commonly used when talking about adding territory to an existing defined area of land , as in “In the twentieth Century the United States annexed the islands of Hawaii.” The best answer, therefore, is “add.” None of the other answer choices are close in meaning to "annex": “accommodate” is  verb that means provide someone with a house or help by making changes to the way in which one regularly does something; "sing" means audibly perform a song; and "sort" means categorize or physically arrange into categories.

The prefix "a-" means not, so “atypical” has the opposite meaning of “typical.” Something that is “typical” is predictable, boring, or common, so “atypical” is closest in meaning to “uncommon.” The prefix "un-" functions here in the same way as the prefix "a-" does in "atypical"; it negates the meaning of the word. As for the other answer choices, “zealous” means passionate; “derogatory” means disrespectful, rude, or offensive; “wary” means cautious, anxious, or careful; and "traitorous" means treacherous or betraying the trust of someone or a group.

The prefix "ex-" means out or away, so to “exhale” means to breathe out, just like to “inhale” means to breathe in. That makes "breathe" the closest synonym to "exhale" amongst the answer choices. As for the other answer choices, "whine" means moan or complain about something.