Anoun that completes an action is called a/an a predicate b complement c subject d object

The correct answer is D. Object. Subject is the noun or the noun phrase that does the action, but the one that completes it is the object. It doesn't have to be a noun but it can be in simpler examples like "I eat apples", where apples is an oun that completes the action of being eaten by Me, who is the subject there.