In a perfectly competitive market, the price of all units of all the goods of that industry will be the same, hence the MR is horizontal as you see. As we know MR is the demand curve and hence the price the producer charges, the producer will sell at MR=MC to maximize profits while at the same time being allocatively efficient. To answer the question as shown, move the bubble to the intersection of the MR and MC that is being touched by the Q line.