The word "obnoxious" is related to the word "noxious." Whenever there are "noxious" vapors in an area, these vapors are unpleasant in odor and perhaps even dangerous. A person who has an "obnoxious" personality is annoying and displeasing to be around. Indeed, such a person can really be horrible company, and that is the sense implied in this question. We tend to think of foolish people as being obnoxious; however, this does not mean that these two words are synonymous with each other. Instead, the foolishness is the cause of the person's being obnoxious. Hence, the only option that really expresses the idea of being unpleasant is "horrible."

The word "deranged" is a very strong adjective for describing something that we think is "totally crazy." Thus, the best option will express this idea of being insane. The only option that does so is "irrational," which means totally unreasonable. For example, you could say, "He was so deranged that he thought he could swim in the lava to save the diamond." This means that the person was so irrational that he thought it was a reasonable thing to save the diamond in this manner.

Whenever someone is "jovial," he or she is very friendly or genial. Such a person is likely very enjoyable company when at a social gathering. It is for this reason that the word "jovial" can have the word "sociable" as a possible synonym. A "sociable" person is likely friendly and very good at interacting with others.

The word "melodramatic" describes someone or something that is overstated in its character. As regards people, a melodramatic person is basically someone who has a dramatic or overstated personality. Thus, the word "overdone" is a fine enough synonym for describing such a person.

Whenever someone is "oblivious," he or she is completely unaware of things. The word is related to "oblivion," which means nothingness. It is like the oblivious person's awareness is filled with nothingness—instead of actual awareness of his or her surroundings! Therefore, we can say, "Henry was so oblivious of his surroundings, that he walked into a wall without even noticing it."

Generally speaking, we use the word "pious" to refer to someone who is religious in a devoted way. In the past, the word used to deal with someone's respect for his or her parents, though this use is much rarer today. Sometimes, "pious" is used to describe someone who is showy with his or her religiousness, though is actually quite nasty and immoral. In this question, however, the only acceptable option is "devout."

Whenever something "wavers," it sways from side to side. If "waver" when you walk, you don't walk a linear path, but instead move from side to side as you proceed forward, winding back and forth.

Whenever we describe someone as being "unwavering," we are saying that he or she is unchanging. This person does not go off the side of the path that has been chosen. Such a person is devoted to what he or she is doing. Hence, "steadfast" is an excellent synonym. It means standing steadily in the sense of meaning unchanging and devoted.

Likely, you know the word "aggressive" in some manner. An "aggressive" person is pushy and generally pretty combative. He or she really does a lot to get his or her way. Aggressive people are not much fun to be around (at least most of the time). So, it's tempting to say that the word means "angry," "enraged," or even "antisocial"; however, being aggressive primarily has to do with being pushy or, as is appropriate given our options for this question, "assertive."

The word "humane" is related to the word "human." In a way, it means acting in a way that befits a true and good human person. Thus, a "humane" person is someone who is understanding and compassionate, not mean and rude. The only option that expresses this general meaning is "understanding." A number of the other options try to trick you by getting you to pick a word that hints at the fact that human beings are very developed creatures. Do not be tricked by these!

The definition of "innate" is possessed at birth or a natural trait. Therefore, the sentence provides the necessary clue for the missing word where it states "babies do not need formal training." The other answer choices do not match this definition.