"Relevant" means appropriate, or "pertinent."

“Obligatory” is an adjective that means “required by a legal, moral, or other rule; compulsory,” so we need to pick out an answer choice that means something like “compulsory.” That means that “voluntary” cannot be the correct answer, because “voluntary” means “acting of one’s own free will,” making it an antonym, not a synonym, of “obligatory.” “Mandatory,” however, is an adjective that means “required by law or rules; compulsory,” and because “mandatory” is the answer choice closest in meaning to “obligatory,” “mandatory” is the correct answer.

"Prime" means main, so "main" is the correct answer. "Secondary" means less important or crucial than something else. "Last" means final. "Unimportant" means not important, as one can infer from the word being a combination of the negative prefix "un-," which means not, and "important." "Average" means common or ordinary.

"Erroneous" means wrong, or refers to something that contains an error.

None of the other words are synonymous with erroneous.

"Opaque" means not clear, blurry, or cloudy.

"Formulaic" means created based on a formula, or standardized.

"Excessive" means too much, or more than is necessary and normal.

"Correct" means right or without error. It is the opposite of erroneous.

"Candid" means honest or frank, and often refers to the manner in which someone is speaking.

None of the other words mean candid:

"Cynical" means thinking the worst about people or events.

"Clear" means apparent or translucent. 

"Convoluted" means confusing or excessively complex to the point of decreasing clarity.

"Deceitful" means dishonest or false.

"Singular" can be synonymous with "unique." The prefix uni- means one single unit, and both these words refer something that is unique or notable in its singular nature. "Plural" is an antonym to "singular." The rest of the words describe things notable for their oddness, while singular or unique things can be odd, this is not always or necessarily the case.

The words "simple" and "quaint" have related meanings describing things or locations that are old-fashioned or charmingly humble. None of the other answer choices have meanings similar to that of " simple." "Complex" describes complicated issues, events, procedures, or ideas. "Taxing" and "demanding" are synonyms describing stressful things.

The words "stellar" and "fantastic" are both positively charged expressions meaning wonderful! The other answer choices do not have the same meaning. The other options all have distinctly negative connotations.

"Credible" means "believable," plausible or rational.

"Authorized" means approved, certified or lawful.

"Corny" means old fashioned, contrived or stupid.

"Fortunate" means having good luck, affluent or prosperous.

"Impartial" means fair and unprejudiced.

"Significant" and "consequential" are adjectives describing things that are important and have meaningful consequences. The other words do not share this meaning. Words like "symbolic" may seem to come close; however, the meaning of "symbolic" is not sufficiently relevant to that of "significant." The remaining words are actually antonyms of the words "significant" and "consequential."