Answer:
b) The average cost must be rising.
Explanation:
Assuming that the entity produce 4 units and its total cost is $16 so average cost per unit is $4 and now the same entity has produced the 5th unit at $5 so the average cost now per unit is (16+5)/5=$4.2
So based on the above discussion, it can be concluded that average cost increase when the marginal cost of production is increased.
So the answer is b) The average cost must be rising.