Solution:Frequent (Adjective) = occussing or done on many occasions; regular.Rare (Adjective) = not occurring very often; occasionally.
Return (very) = go back: give, or put, or send something back to a place or preson.
Fear (Noun) = an unpleasant emotion caused by the belief that someone or something is dangerous, likely to cause pain, or a threat.
Alarm (Noun) = cause someone to feel drightened, disturbed, warning.
Hence, Rare is the antonym of frequent.