"Stiff" means "rigid" or difficult to bend. "Flexible" is an antonym of "rigid"; it means easily changeable or bent. "Delicious" means very tasty. "Boring" means dull or uninteresting. "Selfless" means unselfish or concerned more with others than with oneself.

Someone who is "emaciated" is very thin, to the point of being unhealthy, so "very thin" is the correct answer. "Emaciated" is an antonym of "corpulent," "portly" and "fat."

"Immaculate" means perfectly clean or perfectly behaved, so "spotless," and adjective that also means perfectly clean, is the correct answer. For clarification, "imperfect" means not perfect, as you might guess from the word's use of the negative prefix "im-." It is an antonym of "immaculate." For further clarification, "frank" means honest, "zealous" means passionate, and "vociferous" means vocal or loud.

If someone or something is “emaciated” they are very thin and bony. For example, one could write, "After the famine, the farm animals were “emaciated.” “Gaunt” means thin and bony and usually pale. If you say, "His face looked gaunt," you mean it was thin, pale, and bony. To further help you ,“fibrous” means stringy or made of fibres, so chewy meat is “fibrous.” “Sensitive” means easily offended or sympathetic to other people’s feelings. 

“Strident” is an adjective that is often used to mean harsh and loud. So, we need to pick out an answer choice that means something like loud. That means that “faint” cannot be the correct answer, because “faint” is an adjective that means indistinct or almost imperceptible, making it an antonym, not a synonym, of “strident.” “Audible” cannot be the correct answer either, because while a strident noise is audible (perceptible as sounds via listening), “audible” does not mean the same thing as “strident.” “Piercing,” however, is an adjective that can mean shrill and loud, and because “piercing” is the answer choice closest in meaning to “strident,” “piercing” is the correct answer.

"Lithe" means graceful and limber, like a ballerina, so "graceful" is the correct answer.

“Discrete” is an adjective that means separate, so we need to pick out an answer choice that means something like distinct. That means that “combined” cannot be the correct answer, because “combined” is an adjective that means merged, making it an antonym, not a synonym, of “discrete.” “Individual,” however, when used as an adjective, means separate, and because “individual” is the answer choice closest in meaning to “discrete,” “individual” is the correct answer.

"Rancid" most nearly means rotten, as in "The food that had been left out on the counter top was rancid when the Smiths returned from their two-week vacation."

“Frangible” is an adjective that means “fragile; brittle,” so we need to pick out an answer choice that means something like “fragile.” That means that “shatterproof” cannot be the correct answer, because “shatterproof” means “made so that it does not break easily and will not form sharp, dangerous pieces if it does break,” making it an antonym, not a synonym, of “frangible.” “Breakable,” however, is an adjective that means “capable of breaking or being broken easily,” and because “breakable” is the answer choice closest in meaning to “frangible,” “breakable” is the correct answer.

"Comely" is an adjective that means very cute or attractive, so "cute" is the correct answer. For clarification, "ugly" means aesthetically unpleasant, "imperfect" means having flaws or not perfect, "perfect" means having no flaws or errors, and "boisterous" means loud or unruly.