The correct answer is A. indecent.
Frivolous means not serious or worthwhile. It can also mean lacking in good sense or judgment. Indecent means not decent or proper. It can also mean offensive to morality or modesty.
Serious means having or showing a sober or earnest quality. It can also mean important or significant. Insane means having a severe mental illness characterized by a loss of contact with reality. Rude means impolite or disrespectful.
In this case, the woman is described as being both beautiful and frivolous. This suggests that she is attractive but also lacks good sense or judgment. She may be more concerned with her appearance than with serious matters. This could be seen as indecent, as it suggests that she is not concerned with morality or modesty.
The other options are not as relevant to the description of the woman. Option B, serious, is not a good fit because the woman is described as being frivolous, which means that she is not serious. Option C, insane, is not a good fit because there is no evidence to suggest that the woman is mentally ill. Option D, rude, is not a good fit because there is no evidence to suggest that the woman is impolite or disrespectful.