The correct answer is B. Enthusiast.
An enthusiast is someone who is intensely interested in or excited about a particular activity, object, or concept. An aficionado is a person who is very enthusiastic about a particular activity, interest, or hobby.
A rival is a person who competes with another person or group of people for the same thing, especially in a game or competition. A critic is a person who expresses an unfavorable opinion of something. An indifferent person is not interested or concerned about something.
In this case, the word “aficionado” is used to describe someone who is very enthusiastic about a particular activity, interest, or hobby. The other options do not accurately describe someone who is very enthusiastic about something.