Answer:
The correct answer is option d.
Explanation:
A monopoly firm is a price maker, which means that it decides its price level. Unlike perfect competition where firms are price takers and price is determined by market forces.
So, unlike perfect competition monopoly firms can earn positive profits even in the long run because they can fix the price at a point where the total revenue earned from the sale of the product is higher than the total cost incurred in producing that product.