The last trading day for American options, such as the one held by Xiamen Lee, is typically the third Friday of the month in which the contract is set to expire. This is a standard practice in the United States and most other exchanges around the world. Therefore, since Xiamen's contract is set to expire in September, the last day he can trade his call option is the third Friday of September. It's important to note that while the last trading day is the third Friday of the month, the actual expiration date of the contract is the Saturday following this Friday. However, since trading does not occur on Saturdays, the practical deadline for trading the option is the preceding Friday.\n\nChoice A is incorrect. The third Friday of August cannot be the last trading day for the call option as American options can be exercised at any point until their expiration, which in this case is September. Therefore, it would not make sense for the last trading day to be in August.\n\nChoice B is incorrect. The third Monday of September cannot be the last trading day of the call option because typically, options expire on the third Friday of their expiration month, not Monday.\n\nChoice C is incorrect. Similar to choice A, it would not make sense for the last trading day to be in August when Xiamen's contract expires in September.