When marginal cost is less than average cost, an increase in output ▼ average cost. when marginal cost exceeds average cost, an increase in output ▼ up arrow↑ down arrow↓ average cost?

It's hard to figure out what exactly the question is but the following should cover it: When marginal cost is less than average cost an increase in output lowers the average cost. When marginal cost exceeds average cost an increase in output increases the average cost. This is because you can produce a new item cheaper than what the previous items cost on average. So the new average is lower. The opposite is also true. If the next item costs more than the previous average cost, the new average is higher.