The correct option is C serious
The word 'frivolous' refers to being carefree and superficial.
Option A, indecent refers to something not appropriate of fitting.
Option B, serious refers to acting speaking sincerely and in earnest.
Option C, insane refers to being characterized or caused by madness.
Option D, rude refers to being offensively impolite or bad mannered.
Thus, according to the meaning, the word 'serious' is the opposite of the word 'frivolous'.