Explanation:

- Protean: This word describes someone or something that is able to change frequently or easily, suggesting flexibility and diversity in character or form.

- Option 1: Versatile: Very similar to 'protean', versatile also describes someone who is adaptable and flexible in various functions or activities.

- Option 2: Amateur: Refers to someone who engages in a pursuit for pleasure rather than professionally, and it typically does not have a direct opposite meaning to protean.

- Option 3: Dull: This word suggests lacking interest, excitement, or variety. It is the most opposite in meaning to protean as it reflects a static or unchanging state.

- Option 4: Cautious: Suggests being careful and avoiding risks. It does not inherently contradict the concept of changing easily or frequently.

Your Choice: Option 3: Dull