A trust is the most suitable account type as it provides the maximum level of control desired by the customer. As the grantor of the trust, the customer can establish specific, legally binding terms for how the assets are to be invested and managed by a trustee. Most importantly, the trust document dictates the precise conditions and timing for the distribution of assets to the beneficiaries (the grandchildren), allowing the grantor to maintain control long after the account is established, even beyond the beneficiaries reaching the age of majority.

B. UTMA: In a Uniform Transfers to Minors Act (UTMA) account, the custodian's control ends and assets must be transferred to the beneficiary when they reach the age of majority, which removes long-term distribution control.

C. 529 savings plan: This account is designed for qualified education expenses. Using funds for other purposes results in taxes and penalties on the earnings, thus restricting the customer's control over distributions.

D. Certificate of deposit (CD): A CD is a type of investment, not an account registration. It does not provide a legal framework for managing and distributing assets to a third-party beneficiary.

1. FINRA (Financial Industry Regulatory Authority). "Investment Accounts for Minors." This official FINRA investor education article explains that with custodial accounts like UTMAs, the custodian's control ends when the minor reaches the age of majority. It contrasts this with other options, highlighting the limitations of custodial accounts regarding long-term control. (Accessed under the "For Investors" section on FINRA.org).

2. U.S. Securities and Exchange Commission (SEC). "Investor Bulletin: An Introduction to 529 Plans." SEC Publication. This bulletin clarifies that the primary benefit of 529 plans is for qualified higher education expenses and that "the earnings portion of a non-qualified withdrawal will be subject to federal income tax and a 10% federal penalty tax." (See section: "Withdrawals").

3. Cornell Law School, Legal Information Institute (LII). "Trust." In the Wex Legal Dictionary, the entry for "Trust" defines it as a fiduciary relationship where a grantor gives a trustee the right to hold assets for a beneficiary, noting that the trust is "governed by the terms under which it was created." This supports the principle that a trust provides the highest degree of customized control for the grantor.