The correct answer is A, permissible if the customer provides written instructions. Under FINRA rules, specifically regarding holding customer mail, a firm may hold mail for a customer only if proper written authorization is obtained and certain conditions are met.

Step-by-step, firms must receive current written instructions from the customer specifying the time period for holding mail. While FINRA generally limits mail holds to no more than three months, a six-week (approximately 1.5 months) request is well within the allowable timeframe. Therefore, the duration itself is not a violation.

Choice B is incorrect because prior authorization does not carry forward—each mail hold request requires new written instructions. Choice C is incorrect because the allowable limit is three months, not one month. Choice D is incorrect because opting out of electronic delivery does not prohibit mail holding; customers still have the right to request mail be held.