Answer : d. horses
The direct object of the sentence "I brushed the horses at the stable all afternoon" is the word "horses". A direct object can be a person or a thing that serves as the recipient of the action done in the sentence. Direct objects can be nouns, things, phrases or clauses within the sentence. It is very easy to spot the direct object of the sentence, just bear in mind the formula "subject+verb+direct object". If you were able to determine the subject and the verb in the sentence then, finding the direct object is a piece of cake.
