They knew by his sophomoric remarks that he was still knave in the field.

Answer: Option C

Casual : relaxed and unconcerned.
Unpalatable : not pleasant to taste.
Immature : having or showing an emotional or intellectual development appropriate to someone younger.
Ignorant : lacking knowledge or awareness in general; uneducated or unsophisticated.
Sophomoric : pretentious or juvenile.

Synonym of Sophomoric is Immature