Fill in the blank using the correct tense form:
Proteins ________ carbon, hydrogen, oxygen, and nitrogen.

In the given sentence "Proteins contain carbon, hydrogen, oxygen, and nitrogen," the verb "contain" is in its correct tense form to fill in the blank. It is in the simple present tense, which is used to talk about general truths, facts, or things that are always true.

The sentence is stating a general fact about proteins, which is that they always contain carbon, hydrogen, oxygen, and nitrogen. Therefore, the correct tense form is "contain."

The other options are not suitable for this sentence. Option A, "contains," is in the simple present tense, but it is used for a singular subject, not suitable for "proteins" which is plural. Option B, "containing," is in the present participle form and is not the correct tense for this sentence. Option D, "contained," is in the past tense and does not match the context of the sentence.

Therefore, the correct answer is "contain" (Option C)