Answer:
d. user, buyer influencer
Step-by-step explanation:
A user is defined as someone who utilizes, enjoys and avails of a good or a service.
A buyer on the other hand simply refers to the one who makes the purchases. A buyer may or may not be the ultimate user of the product or a service.
An influencer refers to someone who via his views and recommendations casts a sizable impact on the buyer's ultimate decision.
Thus, in the given case, Stanley is the user of the software, whereas Greg, the purchase manager is authorized to buy it i.e the buyer. Marcus, who has been consulted by Greg owing to his knowledge represents an influencer.