A firm’s average fixed cost is Rs.20 at 6 units of output. What will it be at 4 units of output?

The correct answer is D. Rs.20.

Average fixed cost (AFC) is calculated by dividing fixed cost (FC) by output (Q). Since fixed cost does not change with output, AFC will always be the same regardless of the level of output. In this case, the firm’s AFC is Rs.20 at 6 units of output, so it will also be Rs.20 at 4 units of output.

Option A is incorrect because it is the total fixed cost, not the average fixed cost.

Option B is incorrect because it is the average variable cost, not the average fixed cost.

Option C is incorrect because it is the average total cost, not the average fixed cost.