Many students mistakenly assume restive is related to "rest" and therefore means calm, but it actually behaves more like "restless." A good way to solve this is to recall restive in its typical usage: a "restive horse" or a "restive crowd," both describing agitation, impatience, or resistance to control.
Once this restless, agitated sense is fixed, the search shifts to finding a word describing the opposite state, calm and settled. Among the options, placid is the classic word for calm and untroubled, commonly paired with descriptions of still water or a peaceful temperament, making it the clearest opposite of an agitated, restless state.
The remaining options fail on closer inspection: buoyant describes a cheerful or floating quality, not stillness; resistant is a near homophone trap that actually describes opposition, not restlessness; insolent describes disrespect, an entirely different behavioral trait.
Let's summarize:
Anchoring an unfamiliar word to a common collocation, such as "restive horse," makes recalling its precise meaning much easier under time pressure.