Concept:The verb must agree with the main subject of the sentence, not with a phrase that comes after it.
Explanation:In the sentence, the subject is "The farmer," which is singular.
The phrase "with his sons" is just an extra phrase joined by "with."
Such phrases, including "together with," "as well as," and "along with," do not change the number of the subject.
Therefore, the singular subject "The farmer" must take a singular verb.
So the correct present simple singular verb is "grows."
"Grow," "were growing," and "have been growing" do not agree with the singular subject.
Answer:A. grows