Solution:Frequent (Adjective) = осcurring or done on many occasions or in quick succession; regular).Rare (Adjective) = not occurring very often; infrequently; occasionally, scarce.
Extinct (Adjective) = having no living members; omitted; gone; lost.
Unusual (Adjective) = not habitually or commonly occurring or done: abnormal; uncommon; rare.
Few (Adjective) = some; scant; little.
Hence, rare is the antonym of frequent.