Explanation:

- The phrase "someone working or acting merely for money or other rewards" generally describes a person motivated primarily by personal gain or profit.

- Option 1: Debtor

- A debtor is someone who owes money.

- This term doesn't fit the context of working for money; it's more about owing it.

- Option 2: Mercenary

- A mercenary is someone who is primarily concerned with making money, often regardless of ethics or loyalty.

- This is a one-word substitute for the group of words provided.

- Correct Answer

- Option 3: Contractor

- A contractor is someone engaged in a contract for work but is not characterized specifically by working merely for money or rewards.

- Option 4: Player

- A player typically refers to someone who takes part in a game or sport.

- It's not appropriate for describing someone working merely for money.