Explanation:

- Option 1: Total product increases at a diminishing rate.

- This is correct. When diminishing returns occur, each additional unit of input results in a smaller increase in total output.

- Option 2: Total product increases at an increasing rate.

- This is incorrect. Increasing returns, not diminishing, would lead to total product increasing at an increasing rate.

- Option 3: Marginal product diminishes.

- This is correct. Diminishing returns imply that the marginal product of an input decreases as more of it is added, while other inputs remain constant.

- Option 4: Both (a) and (c).

- This is correct because both option 1 and option 3 accurately describe the characteristics of diminishing returns.

Option 4 is correct.