The correct answer is B. Perilous.
A precarious situation is one that is dangerous or risky. Other synonyms for precarious include dangerous, risky, perilous, hazardous, and precarious.
Brittle is an adjective that means easily broken or damaged. Other synonyms for brittle include fragile, breakable, delicate, and frangible.
Critical is an adjective that means very important or serious. Other synonyms for critical include crucial, essential, vital, and significant.
Cautious is an adjective that means careful and avoiding risks. Other synonyms for cautious include careful, prudent, wary, and circumspect.
In this case, the correct answer is B. Perilous because a precarious situation is one that is dangerous or risky.