The correct answer is: no article.
The word “apple” is a common noun, which means it refers to any apple. In this sentence, we are not referring to a specific apple, but to all of the apples that the speaker bought. Therefore, we do not need to use an article.
If we were referring to a specific apple, we would use an article. For example, if we were talking about the apple that the speaker bought first, we would say “The apple I bought first is sour.”
We would also use an article if we were talking about a type of apple. For example, if we were talking about red apples, we would say “Red apples are sour.”
In conclusion, the correct answer is “no article” because we are not referring to a specific apple or a type of apple.