She is beautiful as well as frivolous.

Answer: Option B

Frivolous : not having any serious purpose or value.
Indecent : not conforming with generally accepted standards of behaviour, especially in relation to sexual matters.
Serious : demanding or characterized by careful consideration or application.
Insane : in a state of mind which prevents normal perception, behaviour, or social interaction; seriously mentally ill.
Rude : offensively impolite or bad-mannered.

Antonym of Frivolous is Serious