Explanation:

- Amateur: A person who engages in an activity for pleasure rather than as a profession, indicating a lack of expertise or professional level of skill.

- Option 1: Hospitable: This pertains to being friendly and welcoming to guests or strangers. It does not relate in meaning to amateur.

- Option 2: Pupil: Refers to a student or learner. While a pupil might be an amateur in their field of learning, this is not a direct antonym for "amateur."

- Option 3: Scholar: Typically, a scholar is learned or knowledgeable in their field. While this indicates expertise, it's not a direct antonym for "amateur."

- Option 4: Professional: Refers to someone engaged in a specified activity as one's main paid occupation, as opposed to engaging in it as a pastime. This directly contrasts with "amateur."

- Correct Answer: Option 4: Professional