We must not take his facetious remark very seriously.

Answer: Option D

Amusing : causing laughter and providing entertainment.
Critical : expressing adverse or disapproving comments or judgements.
Casual : relaxed and unconcerned.
Flippantly : not showing a serious or respectful attitude.
Facetious : treating serious issues with deliberately inappropriate humour; flippant.

Synonym of Facetious is Flippant