Answer:
B) existing carriers prevented from responding to new entrants' lower prices
Step-by-step explanation:
The theory of contestable markets refers to markets with no entry or exit barriers. It was developed by William Baumol. In a contestable market, the number of participating firms is not important. For example, an oligopoly might exist, but if the entry barriers are low they will be forced to act competitively.
What makes existing firms competitive in this type of market, is the risk of new competitors entering the market and reducing their market share. That is why companies will try to make normal profits, because if they are too profitable, lots of potential competitors might enter the market and grab their customers.