Answer:
Option D.
Step-by-step explanation:
Human resources is a term that is used in referring to the people who are employed by a company or the department in a company which is in charge of hiring, training, benefits and records.
In other words, human resources is used to describe the people who constitute the workforce of an organization or industry.
Therefore, in the scenario presented above, John stepping in to resolve the conflict between his workers will fall under human resources because John is dealing with the workforce in his advertising firm.