The correct answer to what are the third gift mentioned in the poem "Love After Love", besides wine and bread, is C. Your heart.
The poem talks about coming back to oneself and loving oneself again. In this passage "Give wine. Give bread. Give back your heart to itself, to the stranger who has loved you", the author makes a list of gifts one can give himself or herself in a moment of reencounter. Thus, the correct answer to which would be the third gift given in the poem "Love After Love" is C. Your heart.
The correct answer couldn't be A. A smile, because that is, according to the poem, how one would greet himself. "And each will smile at the other's welcome". A smile, thus, would be a greeting, not a gift. So, the correct answer couldn't be A. A smile.
The correct answer couldn't be B. Love letters, because the love letters are, in the poem, what one would find when they sat and started feasting on their lives. The letters are already on a bookshelf, they just have to be taken down: "Take down the love letters from the bookshelf". Hence, the correct answer couldn't be B. Love letters, because the love letters aren't gifts.
The correct answer couldn't be D. Photographs, because the photographs in the poem, along with the love letters, are already there to be rediscovered. They had already been left on a bookshelf before. So, they are not gifts, and, therefore, the correct answer couldn't be D. Photographs.