Who is Fowler and what is his first authentic thrill
of the day?

Fowler is a young author, who accompanies Ausable, and wants to learn more about secret agents and everything which goes on, in the life of a secret agent. His first authentic thrill of the day, was encountering Max, who was standing in the middle of Asuable's hotel room, with an automatic pistol in his hand

Fowler was a writer and he had come to meet Ausable. Fowler's first authentic thrill of the day came when he saw a man in Ausable's room pointing a pistol towards Ausable and himself.

b is the answer