Answer:D. Add a formal citation and use the farmer´s explanation in his paper
Explanation: a formal citation is crucial in order to acknowledge that the ideas are not his but they are of someone else and he hasn't stolen them and made them his own ideas.
A formal citation will clearly indicate the author's name and the publisher. Formal citation gives your work an academic credibility , because it means should anyone wants to use the same source of information they will be able to find it .