Answer:
The correct answer is option c. 
Step-by-step explanation:
A competitive firm has been selling its output for $20 per unit and has been maximizing its profit, which is positive. 
The price falls to $18, and the firm makes whatever adjustments are necessary to maximize its profit at the lower price. 
A competitive firm will produce at the point where the marginal cost is equal to price. When the price is lowered the firm will produce at a point with lower marginal cost. 
It will thus produce lesser output than what it was producing earlier. So the quantity of output will be lower than previously.