If you buy something for a really great price, you might say, "es una A. Ganga".
Explanation:
A. is the correct answer because "ganga" in Spanish means a bargain. If you are being offered something for a great price, you would consider this a bargain or a "ganga" because you are saving money.
The other options ("bota" meaning boot, "camiseta" meaning t-shirt, and "gorra" meaning hat) do not make sense in this instance; therefore, A must be the correct answer.