"Instant" is defined as occurring immediately, so "immediately" is the best answer choice! "Delayed" means taking more time to happen than was originally predicted, making it a loose antonym, or opposite, of "instant," and the other answer choices are not related in meaning to "instant."

"Timely" and "opportune" are synonyms. They refer to something that occurs at the right moment. This is a particularly difficult pair of words. If you look at the other answer choices, you'll find that they are all very similar. In fact, the other answer choices are synonymous with each other. "Opportune" is the one word that doesn't fit with the other answer choices. Therefore, it is likely to be related to the word "timely."

The words "stealthy" and "swift" are synonyms describing something that is fast. The other words do not share this meaning. 

The words "stealthy" and "swift" are synonymous. They both refer to something that is quick. The words "slow" and "sluggish" mean the same thing; they refer to something that is not fast. "Loud" and "booming" are synonymous; they refer to something very loud. Identifying this relationship between the answer choices will help you arrive at the correct answer.

"Solitary" is an adjective that means "done or existing alone," or when describing a place, "secluded or isolated." So, we need to pick out an answer choice that means something like "done alone" or "secluded." "Escorted" may look like a good answer, but "escorted" means "accompanied somewhere, especially for protection or security, or as a mark of rank." Because "escorted" means the opposite of "solitary," it cannot be the correct answer. "Unaccompanied," however, means "having no companion or escort." Because "unaccompanied" is closest in meaning to "solitary," "unaccompanied" is the correct answer.

Enormous can be substituted in for vast to maintain a similar meaning.  

"Meager" is an adjective that means "lacking in quantity or quality," and used to refer to something provided or available. So, we need to pick out an adjective that means something like "lacking." While "sufficient" might look like a potentially correct answer, "sufficient" means "enough" or "adequate," "sufficient" is an opposite of "lacking," so it can't be the correct answer. "Paltry," however, is an adjective that means "small or meager" and is used to describe amounts. Since "paltry" is the answer choice closest in meaning to "meager," "paltry" is the correct answer.

“Uniform,” when used as an adjective, means “not changing in form or character; remaining the same,” so we need to pick out an answer choice that means something like “not changing” or “similar to others.” That means that “varying” cannot be the correct answer, because when used as an adjective, “varying” means “differing in size, amount, degree, or nature from something else of the same general class,” making it an antonym, not a synonym, of “uniform.” “Consistent,” however, is an adjective that refers to a person, behavior, or process, and means “unchanging over a period of time,” and because “consistent” is the answer choice closest in meaning to “uniform,” “consistent” is the correct answer.

"Large" could replace "substantial" to maintain the meaning of the sentence. 

"Whole" and "entire" have the same meaning.