The word "gregarious" comes from the Latin for a flock—as in a flock of animals. When we say that someone is gregarious, we mean to say that the person is friendly—capable of being in the "flock" of other people. In contrast to this, an "aloof" person is distant and not very friendly.

If someone is "ostentatious" he or she is very showy—to the point of being quite uncultured in appearance. This generally is done in order to be appreciated and noticed by others. In contrast to this, someone who is "sober" does not merely avoid alcohol (as we often use the word "sober") but above all is very serious and understated in his or her demeanor.

Do not be fooled by the options that all start with "sub-" or "sup-." Although "sub-" is a prefix meaning under, this does not necessarily mean that it should provide the clue for all antonyms beginning with "super-," which means over or above. A supercilious person always looks down on others, as though he or she is better than them. Such a person is arrogant and likely forceful and/or bossy. In contrast to such a person, a meek person is gentle and submissive. This is the best option for an antonym among those given.

The word "loquacious" comes from a rich family of words related to the Latin root for to talk. Such related words are "colloquy," "eloquent," "locution," and "grandiloquent." To be "loquacious" is to be talkative. In contrast to this, a person who is "taciturn" is quiet, speaking few words. A "tacit" agreement is one that is assumed without anything being said explicitly. The words "prolix" and "voluble" are both relatively good synonyms for "loquacious."

The word "cerebral" means intellectual or logical. The opposite of logical is "emotional". "Intelligent" is a synonym of cerebral. "Indigent" means poor and "pompous" means that you feel superior to someone. 

If a person is "pompous," he or she is self-important or overbearing. You might say, "The student body president was overly pompous when she insisted on constantly correcting other officers during the meeting." Someone who is NOT pompous could be said to be "modest," or humble. 

Several of the options try to make you think of "stock" in the sense of "stock market" or the "stock in a store." None of the options, however, function as antonyms even for these meanings of "stock." The word can also mean "ordinary," as in, "He used all of the stock tricks when giving the magic show"—meaning that the magician was not too amazing. It is as though he or she took out the basic tricks from the "stock" that he or she had. Hence, the best antonym for this use of "stock" is "unusual."

The adjective "consummate" describes something that is the best in a given category—or at least one of the best things in that category. If nothing else, it indicates that something is flawless or perfect. The "consummate pianist" is someone who is a great pianist. In contrast to this, the word "flawed" is an excellent antonym.

Something is called "economical" when it is priced in a way that gives the most value per dollar. For instance, a car that is cheap, small, gets good gas mileage, and requires little maintenance is quite economical. In contrast to this, something exorbitant is very expensive—often to the point of making little financial sense. While we might think of better antonyms than this, of the options provided, this is the best match, for the others do not function as antonyms in any proper sense. (However, some options like "familial" and "laborious" attempt to trick you into thinking these things are contrasted to the "economy" in the sense that we normally use that word.)

Something that is "mediocre" is merely "okay" in its quality—often even implying that it is not of very good quality at all. A number of the options provided are meant to trap you into a wrong answer. Something that is excellent—the opposite of mediocre—might indeed be "surprising," "famous," or "unexpected." However, the best option is "exemplary," a word which describes something that is very excellent—to the point of being an "example" or standard for other things of a similar type. This best captures a direct opposition to mediocrity.