Explanation:

- Total variable costs refer to costs that change with the level of output (like raw materials, direct labor).

- Option 1: increase

- Correct. Total variable costs always increase as output increases, because more production means more variable inputs.

- Option 2: increase at an increasing rate

- Sometimes true if there are diseconomies of scale, but not always.

- Option 3: increase at a constant rate

- Only true if variable cost per unit is constant. Not always the case.

- Option 4: increase at a diminishing rate

- Only true if efficiency improves with scale, which is not always the case.