Average fixed

a. equals marginal cost when average total cost is at its minimum.
b. may be found for any output by adding average variable cost and average total cost.
c. graphs as a u-shaped curve.
d. declines continually as output increases.

d. declines continually as output increases.

Explanation:

The reason for this is that when because fixed cause remains the same as output increases the average fixed cost decreases when output increases. For example the fixed cost of a factory is 10,000 and it produces 100 units. In this case we will divide the fixed cost by the number of units to find the average fixed cost. 10,000/100=100

Now when we increase output to 200 the average fixed cost will decrease.

10,000/200=50.

Mathematically we can view this as the numerator is staying constant whereas the denominator is increasing when output increases, therefore average fixed cost is declining.

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the third answer is correct.