Answer: over the counter
Explanation: In simple words, over the counter markets are the platform in which the securities like foreign exchange are traded without any supervision. There is no monitoring authority in such markets as opposed to the structure of stock exchanges.
Foreign currencies are traded in over the current markets as these are highly fluctuating in values and the margins are very low in trading such securities. Therefore, to pay a mediator is not feasible while dealing in foreign currencies.
Hence from the above we can conclude that the correct answer is option B.