"Understated" means modest and not drawing much attention. To be "subtle" means to be subdued and not obvious. Thus, given how similar in meaning that these words are to one another, they are synonyms.

Anything that is "vital" is absolutely required for a certain process or goal. For example, food and water are vital ingredients for human survival. "Necessary," or needed, is a strong synonym of "vital," while both "powerless" and "agreeable" are unrelated. Finally, perhaps you are unfamiliar with the final choice, "superfluous." This word simply means extra or unnecessary, which is the exact opposite of the choice we want to pick.

“Intense” means to a great degree or "extreme." The word “moderate” means in the middle of two sides or poles, whereas “mediocre” means of average quality when higher quality was expected. Something that is “effortless” does not require effort or work or is extremely easy to do, and something that is "mild" is not very strong or serious.

"Vital" derives from the Latin "vita," life, so it makes sense that something that is “vital” is very important to keeping life going, or more generally, necessary and very important. "Necessary" is thus the correct answer. As for the other answer choices, “extraneous” means not needed or part of an amount that is more than what is needed; “aggressive” means angry and violent or intense; “passive” means not doing anything in response to something; and “fatal” means deadly.

"Trifling," when used as an adjective, means "unimportant or trivial." So, we need to pick out an answer choice that means something like "unimportant." "Important" is the opposite of "unimportant," so it can't be the correct answer, but "petty" means "of little importance; trivial," "characterized by an undue concern for trivial matters, esp. in a small-minded or spiteful way" when used to describe behavior, or "of secondary or lesser importance, rank, or scale; minor." Because "petty" is the answer choice closest in meaning to "trifling," "petty" is the correct answer.

Hearty means strong and healthy. To further help you, wise means very smart, able to make good decisions; benevolent means good-natured; malevolent means wicked-natured; benevolent and malevolent are antonyms of one another; visible means able to be seen.

Something "extreme" is something that exists to a high degree. For example, extreme heat is very hot—at a high degree of heat. Extreme anger is a large amount of anger. The general idea of being extreme is this kind of largeness or greatness of amount. Thus, the best option is "enormous." None of the other options are synonymous with the idea of being of a large size or to a great degree.

"Virulent" means strong, extremely lethal and poisonous. The closest answer then is "toxic" which means poisonous. To provide further help, "beneficial" means helpful; "obsolete" means no longer used, no longer needed.

When you "improve" something you make that thing "better." So these two words are clearly synonyms. "Worse" is an antonym to improved. "Friendlier" and "calmer are both statements of improvement but apply only to a person's (or animal's) disposition.

The word "infinite" actually means not finite. Now, if something is called "finite" it is measurable. In other words, it "comes to an end." Thus, even though a billion is a large number, a billion dollars is finite. You can eventually "come to an end" when counting it; however, something "infinite" is endless. For example, there are an infinite number of numbers when you count. You can always keep counting one number higher. Thus, the best option is "limitless" meaning "having no limit" or "having no endpoint."