Final answer:
The requirement for an insurance agent not to mix their insurance funds with personal funds is known as a fiduciary duty. This duty is fundamental to maintaining trust with clients and ensuring the proper handling of their funds.
Step-by-step explanation:
The principle that an insurance agent must not commingle their insurance funds with their personal funds is referred to as a) Fiduciary duty. This legal requirement is based on the agent occupying a position of trust on behalf of their clients, where they are expected to manage and safeguard client funds separately from their own. Mixing these funds, or commingling, would breach this trust and violate the fiduciary duty they owe to the clients. Options b) Collateral estoppel, c) Tortious interference, and d) Ultra vires, do not relate to this concept of non-commingling of funds.