The word "porous" comes from the same roots as the word "pore"—yes, meaning the things on your skin! You can think of pores as kinds of holes in the surface of the skin. Therefore, the general meaning of "porous" is full of small holes that let things pass through. Thus, a porous surface can be called "permeable," which also describes something that is not locked up but, instead, lets certain materials through. A "permeable membrane" is a kind of membrane that allows water and other substances through it.

The general idea of something being "absolute" is of something that is unmoving and unchanging. This is opposed to being "relative." We speak of "'relative" matters when we say things like, "The desk is tall, at least in comparison to the pea on the ground," and "George is more handsome than Harold." However, the word "absolute" can then be extended to mean "unlimited." Whenever something is "unlimited," there is nothing relative to it that can limit its exercise or force. Thus, "absolute power" is unlimited power that has no check on it whatsoever.

The word "perceptive" clearly is related to "perception" or "to perceive." When we perceive something, we either sense it or understand it. For example, you might say, "He perceived the danger of the situation," meaning that the person understood that the situation was dangerous. A "perceptive" person is someone who has the ability to understand. Hence, he or she could well be called "insightful," indicating his or her clear "vision"—meaning that he or she has a clear ability to understand things. Likewise, we can describe something like remarks as being "perceptive," meaning that they express matters intelligently.

Whenever something is "habitual," it has become a fixed and regular habit. For example, someone might habitually tap his foot, meaning that he regularly taps it no matter where he is at. Likewise, a person who regularly smokes cigarettes is rightly called a "habitual smoker." The word can be used to describe any settled way of acting; hence, the word "customary" is a good option for this question. "Customs" are the kinds of things that we establish in our culture and just "keep doing" without thinking too much. Hence, there are customs like saying "thank you," but also customs like buying flowers for those whom we love. We do these things out of a kind of habit. Hence, the word "customary" is a good option among those provided here.

Probably the most tempting wrong answers for this question are "reservedly" and "timidly." When someone is "reserved," he or she is quiet or at least does not put him or herself forward to express things. Likewise, a "timid" person is fearful and thus often quite shy. Such a person is likely quiet. The best option for a synonym here is "inaudibly." When something is "audible," it is able to be heard. (Audio, for example, is something that you listen to. The root words for the two words are the same.) The "in-" prefix is called a "privative" prefix. It means not. Therefore something that is in-audible is not audible—silent!

"Ideal," when used as a noun, is defined as a standard of perfection or excellence, so "standard" is the best choice. Be careful not to confuse "ideal" with "idea." None of the other answer choices are close in meaning to "ideal": "scheme" means plot; "perception" means the act of recognizing something using one's senses; "feat" means accomplishment; and "bulletin" means report or news story.

The word "horrid" means horrible or terrible. The best choice is "terrible" as it shares the same definition as "horrid."

"Scarce" means rare or in short supply, so "rare" is the correct answer. To help you, "abundant" means plentiful, "common" means normal or regularly encountered, "rout" means beat or thrash, and "admonish" means criticize, tell-off, or scold. 

"Scarce" means few in quantity. The word "rare" is the best choice because it means not common or infrequent. Considering the other choices, "shameless" means without shame, "gleeful" means very happy, and "spacious" means large in area.