The correct answer is A, The customer is the RR’s sister. Under FINRA rules (Rule 3240), borrowing from or lending to customers is generally prohibited, unless specific exceptions apply and firm approval is obtained. One key exception is when the customer is an immediate family member, which includes relationships such as parents, children, siblings, and spouses.

In this scenario, since the customer is the RR’s sister, the borrowing may be permitted, provided the firm’s policies allow it and proper written approval is obtained. This type of relationship reduces the risk of exploitation or conflict of interest that the rule is designed to prevent.

Choice B, coworker, is incorrect because simply working together does not qualify as an exception under FINRA rules. Choice C, a letter of instruction, is also incorrect because written permission from the customer alone does not override regulatory restrictions—firm approval and qualifying conditions are required.

Choice D is incorrect because while borrowing is generally restricted, it is not absolutely prohibited in all cases—there are limited exceptions, such as immediate family relationships.

Thus, borrowing is permissible in this case due to the familial relationship, making choice A correct.