The correct answer is: Proteins contain carbon, hydrogen, oxygen, and nitrogen.
The other options are incorrect because they are not in the correct tense form. Option A, “contains,” is in the present tense, but the question is asking for the correct tense form of the verb “to contain” in the present tense. Option B, “containing,” is in the present participle tense, but the question is asking for the correct tense form of the verb “to contain” in the present tense. Option C, “contain,” is in the plural form, but the question is asking for the correct tense form of the verb “to contain” in the singular form. Option D, “contained,” is in the past tense, but the question is asking for the correct tense form of the verb “to contain” in the present tense.
The verb “to contain” means to have something inside of something else. In the case of proteins, they contain carbon, hydrogen, oxygen, and nitrogen. These elements are essential for the structure and function of proteins.