a. vegetables.
This is the direct object in the sentence.
Direct object is a noun or pronoun that receives the action of the verb.
It answers the questions what? and whom?
In this case, the verb is grows. What grows? Answer: vegetables.
The verb that is used in this sentence is called transitive verb. A verb is only called transitive when it is; 1, an action verb and 2, it has a direct object.