Correct Answer: Amateur

Solution : The correct choice is the fourth option .

• Professional refers to doing something as a paid occupation or career.
• Amateur refers to doing something for pleasure or personal interest rather than as a profession.
• Immature and mature are terms that generally describe levels of development and are not directly related to doing something professionally or for pleasure.

Therefore, for the specific context of doing something for pleasure but not as a profession, the appropriate one-word substitute is amateur.