Explanation:

- "Amateur" refers to someone engaging in an activity for pleasure rather than for professional reasons. It's a non-expert or someone not fully proficient in a specific field.

- Option 1: "Apprentice" typically describes someone who is learning a trade from a skilled employer. They are still in training but are working towards becoming a professional.

- Option 2: "Novice" signifies someone new or inexperienced in a particular activity or field. It closely aligns with "amateur" as it also denotes a lack of full expertise.

- Option 3: "Learner" is a general term indicating someone who is in the process of acquiring knowledge or skill and does not imply expertise.

- Option 4: "Professional" is the most appropriate antonym of "amateur." It defines someone who is fully skilled and usually paid for their expertise in a field.