Which of the following bank customers cannot buy units of a common or collective fund?

A checking account customer typically does not have the authority or the mechanism in place to directly invest in mutual funds or collective investment schemes. Their relationship with the bank is primarily for managing transactional banking needs, such as deposits, withdrawals, and routine payments. While they may have the option to invest in various financial products through the bank, a checking account itself does not provide the capability for purchasing investments, such as units of a common or collective fund.

In contrast, the other types of customers listed, such as personal trusts, employee benefit trusts, and corporate accounts, often have the necessary structures or purposes that align with investment in such funds. These accounts are typically designed to manage investments for specific goals, like growth or retirement benefits, and they have the legal and financial framework to engage in such transactions.