"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."

The word "intense" is used to describe something that is very strong or very extreme. For example, "intense anger" is a lot of anger, "intense heat" is very hot, and an "intense smell" is one that is very strong and smelly. Thus, the best option among the options provided is "extreme." This word also means a lot. It is fair to say that "intense heat" is also "extreme heat."

The word "ferocious" is related to the word "fierce." This can mean extreme, as when we say, "There was a fierce storm last night." This means that the storm was strong.  Often "ferocious" is used to describe animals that are very fierce and dangerous.  Thus, it can also mean "wild."  For example, to say, "The ferocious dog bit the burglar's leg and would not let go," means that the dog was fierce in its attack on the burglar.

The word "feeble" describes someone who is very weak.  It also can describe something physical that is not very powerful. For example, a "feeble sound" is one that is very soft. When we say that someone gave a "feeble attempt" at something, we mean that he or she really didn't try very much. While it may be tempting to say that someone who is feeble is also lazy, this really is not fair. The only thing we can say about the word "feeble" is that it means "weak".  This is the best answer for this question.

We often use the word "mild" to describe food. Mild foods are not spicy; however, the word "mild" can also be used to describe things that are not harsh in general. Thus, a "mild temperament" is one that is gentle. Also, a scolding can be called "mild."  In this case, you mean something like light. If your mother only yells at you a little bit, that would be a mild scolding. That is the meaning being used here.

The word "potent" actually comes from roots that mean able or having the power to do something. A "potential boxer" is someone who has it in his power to become a boxer. We tend to think of "potions" as being toxic. Do not get confused with these words. (Several of the options might lead you to do this.) "Potent" merely means "powerful" or "strong." The latter is the correct option for this question.

"Frailty" has to do with being weak. A "frail" person is someone who is lacking strength. It can also be used to describe something that is "fragile"—easily broken. While "elderly" people are often frailer than young people, the options "aged" and "elderly" are traps. First and foremost, frailty has to do with lacking strength. Thus, the best option is "weak."

Whenever we have "acute pain," it is a severe and sudden kind of pain that we experience. Do not be tricked in this question, however, into thinking that either "emergency" or "medical" are synonyms. Likewise, don't let your math knowledge trick you! An acute angle is one that is less than 90 degrees. "Acute" is a synonym neither for "mathematics" nor for "triangle"!  The simple word "severe" is the best!