Antonyms

Impetuous means hasty; rash; impulsive; unplanned. Its opposite is planned.

Implausible means impractical; outlandish. Its opposite is extremely practical.

Immune means protected (from disease or poison); not susceptible. Its opposite is susceptible.

Hackneyed (in relation to a phrase) means overused; common; trite; platitudinous. Its opposite, among the choices, is fresh.

Another antonym of hackneyed is original.

The opposite of halcyon, among the choices for this question, is miserable.

Halcyon means calm and peacefully happy; serene. Its opposite, among the choices, is agitated.

Gossamer means thin; flimsy. Its opposite is ponderous, meaning heavy; bulky; weighty.

Garrulity means talkativeness. Its opposite is taciturnity, meaning silence.

Feral means untamed; wild. Its opposite is domesticated.

Fetid means foul-smelling. Its opposite is having a pleasant smell.

Fervid means enthusiastic. Its opposite is restrained.

Flamboyant means showy; flashy; ornate; lavish. Its opposite is subdued, meaning modest; unpretentious.

Flippancy means lack of seriousness.