"Tangential" means straying from a particular subject or topic of conversation. The best choice is "off-topic" because it shares the same definition of "tangential." Considering the other choices, "triangular" means resembling a triangle or having the shape of a triangle, "mysterious" means unclear or unexplainable, and "pedestrian" means unoriginal.

"Clandestine" is an adjective that means secret, so "secret" is the correct answer. To provide further guidance, "contemporary" means modern or of the relevant time period, "mundane" means boring, "ideal" means perfect, and "idle" means lazy or motionless.

"Earnest" means honest or sincere, so "sincere" is the correct answer.

“Fatal” means deadly or lethal. To provide further help, “unruly” means not obedient, naughty; “dominant” means in charge, in control; “feeble” means weak and pathetic; “robust” means strong and sturdy. Someone who is “feeble” is opposite to someone who is “robust,” so the two words are antonyms.

"Genuine" means real or authentic, so "authentic" is the correct answer. None of the other answer choices are close in meaning to "genuine": "generous" means willing to give time or money to another person or cause; "thrifty" means frugal or spending carefully; "contagious" means capable of being spread easily and is used to describe diseases; and "stalwart" means reliable and loyal.

"Frauduent" means fake or deceptive, which is closest to "counterfeit."

A “fraud” is a phony, so it stands to reason that something or someone who is “fraudulent” is fake. To provide further help, “garrulous” means talkative; “genuine” means real, authentic and is an antonym of “fraudulent”; “reserved,” when used to describe a person, means shy; “wanton” means random and senseless. 

"Tentative" means unsure or uncertain. For example, one might write, "Gina had tentative plans to visit her mother that evening, as she was still waiting to hear whether her mother wanted to spend time with her." So, "uncertain" is the correct answer.

The word "demonstrative" refers to something that is meant to explain some information. The best choice is "explanatory" because it shares the same definition as "demonstrative." Considering the other choices, "achievable" means able to be attained, "lacking" means not having, and "showy" means making a great display. 

"Didactic" means serving to instruct or teach, so "instructive," which means serving to instruct, is therefore closest in meaning. 

Something that is didactic may be "interesting," but this is possibly a characteristic of something that is didactic, rather than a definition. Therefore, "interesting" cannot be the correct answer. As for the other answer choices, "dynamic" means active or  changing and "candid" means honest.