The correct answer is C. no article.
The word “salt” is a non-count noun, which means it does not have a plural form and cannot be preceded by an article. Non-count nouns refer to things that cannot be counted individually, such as water, sand, or air.
In the sentence “Have you put salt in the soup?”, the word “salt” is referring to salt in general, not a specific amount of salt. Therefore, it does not need an article.
If the sentence were asking about a specific amount of salt, such as “Have you put a teaspoon of salt in the soup?”, then the article “a” would be necessary.
The other options are incorrect because they are all articles. The article “the” is used to refer to a specific thing that has already been mentioned, while the articles “a” and “an” are used to refer to a general thing or a thing that has not been mentioned before.