If you are "dubious" about something, you are unsure or doubtful about the outcome. For clarification, "serendipitous" means owed to good fortune or unexpected, "dilatory" means slow or lacking, "inundate" means overwhelm or flood, and "remedy" means to fix.

"Candid" means truthful or honest, so "honest" is the correct answer. "Deceitful" is an antonym of "candid" and "honest"; it means dishonest or trying to trick someone. "Gigantic" means extremely large. "Persuasive" means very convincing. "Injured" means hurt or having an injury.

“Dubious” means questioning or questionable. If a person is “dubious” about something, they are uncertain or unsure. To help, "cumbersome" means heavy or hard to manage. So, for example, full suitcases could be cumbersome. To provide further help, "regretful" means wishing that you had either done something you did not do or not done something you did. 

"Paramount" means upmost or supreme, as in the commonly heard phrase, “of paramount importance.” For clarification, "mediocre" means average in a negative way; "penitent" means feeling regret for sinful behavior; "outlawed" means banned or criminalized.

"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.