How often? (option "A") is used in question forms so as to get an adverb answer, such as an adverb of frequency. The adverbs of frequency are used to express the frequency that something happens; for example: always, often, hardly ever, etc. The adverbs of frequency are considered adverbs of time, and they can be used to describe how often an action occurs, either in a definite frequency (weekly, daily, yearly) or in an indefinite one (sometimes, often, rarely).
