Final answer:
Typically, personal trust customers, employee benefit trust account customers, and checking account customers can invest in common or collective funds like mutual funds unless specific circumstances or regulations prevent them from doing so.
Step-by-step explanation:
The question of which bank customers cannot buy units of a common or collective fund, such as a mutual fund, touches on the eligibility and restrictions that financial institutions may have for their different types of customers. A personal trust customer, an employee benefit trust account customer, and a checking account customer are typically eligible to invest in common or collective funds. These types of investment vehicles are designed to be accessible to a broad range of investors, and there's no inherent reason within the scope of standard banking practice why these customers could not buy into such funds. Instead, ineligibility would usually be based on the specific circumstances of the customer, such as regulatory restrictions, the customer's risk profile, or the specific terms set by the financial institution.