The correct answer is the speaker's lover.
Sonnet 73 is one of Shakespeare's most famous sonnets; however, it does not deal with love, as most of his sonnets do. This one talks about old age and the narrator's fear of it. He is addressing the Fair Youth, a young man who is thought to be the narrator's lover.
Therefore, the implied listener cannot possibly be the father or the child, and teacher is not even mentioned in the sonnets; the implied listener is actually the lover whom the speaker is urging to take advantage of his youth before it is too late.
