Clearing houses are always used in futures markets and sometimes used in OTC markets. A clearing house is an intermediary between buyers and sellers of financial instruments. It is always used in regulated (exchange) markets to settle trading accounts, clear trades, collect and maintain margin monies, regulate delivery, and report trading data. Futures contracts are exclusively traded in organized exchanges, hence the constant use of clearing houses. Although OTC markets (decentralized markets in which market participants trade stocks, commodities, currencies, or other instruments directly) traditionally do not involve a clearing house, they are increasingly embracing the idea of central clearing through central counterparties (CCPs) which are a type of clearing house. Therefore, clearing houses are sometimes used in OTC markets.

Choice A is incorrect. Clearing houses are always used in futures markets, not never. They act as the counterparty to both sides of a futures contract - buyer and seller - reducing the risk of default by either party. However, they are sometimes used in OTC markets, which is correctly stated.

Choice C is incorrect. While clearing houses are always used in futures markets, they are not always used in OTC markets. The use of clearing houses in OTC markets depends on various factors such as the type of financial instrument being traded and the regulatory environment.

Choice D is incorrect. This choice incorrectly states that clearing houses are never used in futures markets when they actually play a crucial role there by acting as intermediaries between buyers and sellers to reduce default risk. Also, it's wrong to say that clearing houses are always used in OTC markets because their usage varies depending on several factors.