The correct option is B serious
Frivolous means not having any serious purpose or value. The opposite of frivolous is serious or demanding or characterized by careful consideration or application. Hence, Option A is correct. Among the rest of the options, flippant means not showing a serious or respectful attitude, frantic means distraught with fear, anxiety, or other emotion, fanciful means over-imaginative and unrealistic. These options do not express this antonym, hence incorrect