In a remedial practice session, the fact pattern is framed this way. Assume the administrator is testing the cleanest state-law answer. A broker-dealer deposits a customer's funds into the firm's general operating account. This is an example of:

Why B — Commingling of funds, which is prohibited

Under NASAA model rules and USA Section 502, commingling customer funds with a broker-dealer's own funds is a prohibited practice. Customer funds must be maintained in separate accounts to protect them from the firm's creditors and to ensure proper accounting. Mixing customer and firm funds creates significant risks of misuse and loss. State-law analysis leads to the same conclusion despite the alternate scenario.