Explanation:

- A trade that is prohibited by law requires one word to capture the essence: an act that is illegal or forbidden.

- Option 1: illicit

- The correct answer.

- Refers to something forbidden by law, rules, or custom.

- Option 2: inapt

- Means not suitable or appropriate in the circumstances.

- Option 3: illusive

- Refers to something that is deceptive or misleading.

- Option 4: incredible

- Means difficult to believe; extraordinary.