Step 1: Understand the context of the question.
The question is about the conditions under which a buyer can fail to fulfill a foreign exchange (forex) transaction contract.

Step 2: Identify the possible reasons a buyer might not fulfill a forex contract.
In financial markets, contracts are binding agreements、However, there are specific circumstances or clauses that might allow a party to back out or not fulfill their obligations.

Step 3: Recognize the specific condition related to forex contracts.
One key condition that allows a buyer to not fulfill a contract is if the contract includes a clause that permits cancellation or nonfulfillment under certain circumstances.

Step 4: Recall the term associated with the condition.
The term 择期外汇买卖 translates to optional forex trading or more specifically, it relates to the concept where one party has the right but not the obligation to fulfill the contract under certain conditions.

Step 5: Match the condition to the given options or context.
Given the context, 择期外汇买卖 (optional forex trading or more accurately, a type of forex transaction that allows for flexibility) is related to the concept where a buyer has the flexibility or option regarding the fulfillment of the contract.

Step 6: Provide the specific term or condition.
The condition is related to 择期外汇买卖合约 which implies a type of contract that allows for certain flexibility.

The best answer is B.