Which statement about direct objects is true?
a. they are found only in sentences with action verbs.
b. they must be placed before the verb phrase.
c. they are considered a kind of complement only when telling how many.
d. they follow a linking verb and rename the subject of a sentence.

The true statement about direct objects is A. THEY ARE FOUND ONLY IN SENTENCES WITH ACTION VERBS.

Direct objects are called such because they are the recipient of the action of the verb. 

A direct object is either a noun or pronoun that receives the action of the verb or shows the result of the action. It is found after the verb or indirect object. It answers the question "what" or "whom".  The verb used in a sentence with a direct object is called "transitive verb".