The correct answer is C. Fashion.
A fad is a craze or custom that is followed for a short time, especially one that is taken up enthusiastically by a particular section of society. Fashion is the prevailing style in clothing, hair, and other aspects of personal appearance at a particular time.
Charity is the voluntary giving of help, typically in the form of money, to those in need. Weakness is a lack of strength or power. Folly is foolishness or stupidity.
C is the most similar to fad because it is a temporary trend that is followed by a large group of people. Charity, weakness, and folly are not temporary trends.