The word "senescent" is related to roots from which we get words like "senior" and "senator," the latter really describing a person who is supposed to be old and wise. To be "senescent" is to be aged, often with the effects of deterioration that come with the passage of time. Although the word "adolescent" can be a bit negative, it does not have to be such. It can merely mean that someone is in the period of youth in which he or she is passing to adulthood. Among the options provided, this is the best temporal contrast to "senescent."

When someone "plods along," he or she moves at a slow speed, often as though he or she has a heavy burden on his or her back. Do not be tempted to pick an answer like "joyous." Just because someone is plodding, he or she does not necessarily do this because of being depressed or sad. The best antonym is "expeditious," meaningdone quickly and efficiently. To "expedite" a process is to make it happen more quickly.

The word "inanimate" comes from the Latin root meaningto have a soul. We generally refer to things as being "animated" when they move, seeming to have self-motion. An "animate" being is one that is living. The most tempting wrong answer given for this question is "biotic," which meansrelating to living things; however, this does not describe the living thing itself. The best answer—as homely as it might seem—is "living."

The word "precipitate" is most often associated with the phenomenon of meteorological precipitation—as when it rains or snows. This is a specific case of the more general process of something falling out of solution or suspension. We can say that a person makes a "precipitate decision" when he or she makes a quick and unmeasured decision—proceeding as quickly as the dry skies begin to fill with rain. Thus, such a hasty decision is well contrasted one that is thought out and measured—i.e. to a prudent decision.

Something that is protracted is drawn out or lasting for too long of a time. Do not associate this word with the mathematical/geometrical options given in the question—all of these are trying to get you to make a false association. In contrast to being "protracted," "brief" is an excellent antonym. A "protracted affair" is one that is too long, but one that is brief is quite short in contrast.