The correct answer is C. The law of one price states that identical goods should have the same price in different markets when there are no transportation costs or trade barriers. In order for this to hold, transaction costs associated with arbitrage, which is the process of buying a good in a cheaper market and selling it in a more expensive market, must be zero. If transaction costs exist, then arbitrage will not be profitable and the law of one price may not hold.
The other options listed are not directly related to the law of one price. Antitrust laws, arbitrage legality, yield management, and barriers to entry can affect competition and market outcomes, but they do not have a direct impact on the law of one price.