The correct answer is: D. nothing
The sentence “My stomach is full. I won’t have anything else” means that the speaker is not hungry and does not want to eat anything else. The word “anything” is used to refer to all possible things, so it means that the speaker does not want to eat anything at all.
The other options are incorrect because they do not accurately reflect the meaning of the sentence. The word “little” means a small amount, so it does not make sense to say that the speaker does not want to eat a small amount of food. The word “something” means an unspecified thing, so it does not make sense to say that the speaker does not want to eat an unspecified thing. The word “a little” is a combination of the words “a” and “little”, so it means the same thing as the word “little”.