On settlement date, a customer is unable to pay for a purchase in his cash...

In a cash account, a customer must pay for securities by the required payment date. If the customer fails to pay and the broker-dealer liquidates the position, the customer’s cash account is restricted for 90 days under Regulation T. During the restricted period, the customer may still purchase securities, but only if sufficient settled cash is in the account before the trade is accepted. Choice C is correct. Choice A is incorrect because the customer is not limited only to closing transactions; rather, purchases must be fully paid in advance during the restriction period. Choice B is incorrect because the restriction period is 90 days, not 30 days. Choice D is too broad because the question asks about the customer’s account, not automatically all related accounts. The SIE outline includes account types, margin and cash account concepts, trade settlement, and Federal Reserve Board Regulation T. This question tests cash account payment failure and the consequence of freeriding or failure to pay in a timely manner. Reference: Understanding Trading, Customer Accounts and Prohibited Activities; Customer Account Types; Trade Settlement; Federal Reserve Board Regulation T.