Explanation:

- Total Revenue (TR) increases at a constant rate when a firm sells additional units at a consistent price.

- Marginal Revenue (MR) is the additional revenue earned by selling one more unit.

- Option 1: Increasing

- If MR is increasing, it would indicate that each additional unit sold yields more revenue than the previous one. This contradicts TR increasing at a constant rate.

- Option 2: Decreasing

- If MR decreases, each additional unit adds less revenue, which conflicts with constant rate of TR increase.

- Option 3: Constant

- If TR increases at a constant rate, selling each additional unit adds a fixed amount to the revenue, making MR constant.

- Option 4: Zero

- MR being zero suggests that selling additional units doesn't change TR, opposing the notion of increasing TR.

Option 3: Constant is the correct answer.