A new client is opening a Registered Retirement Savings Plan.

When a financial institution such as a bank or an investment advisor collects a social insurance number (SIN) for tax reporting purposes, especially in contexts like opening a Registered Retirement Savings Plan (RRSP), it is mandatory because the SIN is required by law for such purposes. However, the use of the SIN for other purposes, such as identity verification, is not mandatory and should only be done with the client's consent. Thus, the correct answer is A, "Optional for identity verification purposes," indicating that while the SIN is necessary for tax purposes, its use for verifying identity is at the client's discretion.