Which of the following describes a control agreement in relation to deposit accounts?

A control agreement in relation to deposit accounts is indeed characterized by a requirement for the bank to operate only under the instructions of the secured creditor. This means that in situations where a creditor takes a security interest in a deposit account, a control agreement must typically be established to ensure that the bank recognizes the creditor's claim on the account.

The function of a control agreement is to grant the secured creditor the ability to direct the bank in regards to the deposit account, which includes the disbursement of funds or other account-related transactions. This arrangement is critical for the creditor to have enforceable rights over the account, ensuring that they can exercise control over the funds if the debtor defaults.

In contrast, the other options do not accurately describe control agreements. For example, a control agreement does not guarantee a fixed interest rate or allow unrestricted access to funds. Additionally, while a creditor being a customer might simplify certain aspects of the banking relationship, it does not negate the need for a control agreement to establish the creditor’s rights over the deposit. The requirement for creditor instructions is what fundamentally distinguishes a control agreement in secured transactions involving deposit accounts.