誰がこの手紙を書きましたか。

who

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

this (something or someone close to the speaker (including the speaker), or ideas expressed by the speaker)

letter

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to write; to compose; to pen; to draw; to paint

indicates a question (sometimes rhetorical); or; whether or not; some- (e.g. something, someone); indicates doubt, uncertainty, etc.; emphatic prefix; suffix forming adjectives or adverbs; in that way