D. Not only the muffler but also the spark plugs need to be replaced
Among the sentences presented above, this statement shows a correct subject and verb agreement. As you can see, the subject for this statement are " the muffler" and "the spark plugs" which means to say that the subject is in plural form. In the subject verb agreement (SVA), it states that the singular subject must take the singular form of the verb while plural subject must take the plural form of the verb. In the statement, the verb is "need", and need is the plural form of the verb "need" in its base form. Therefore, the statement follows the correct SVA format.