The correct answer is B.

Pledging requirements refer to the legal obligations that require securities to be pledged as collateral for specific deposits. In the context of depository institutions in the United States, these requirements are particularly relevant when dealing with deposits from federal, state, and local governments. These institutions are not allowed to accept such deposits unless they post collateral that is deemed acceptable by these governmental units. This is done to safeguard public funds and ensure that the government's interests are protected in the event of a default. The nature and extent of these pledging requirements can vary widely from state to state. However, most states allow a combination of federal and municipal securities to meet these requirements. Therefore, the term 'pledging requirement' accurately encapsulates this regulatory obligation imposed on depository institutions.

Choice A is incorrect. Statutory requirements refer to the laws that a company or an institution must follow. These are broader in scope and not specifically related to the requirement of providing security in the form of pledged collateral for accepting deposits from government units.

Choice C is incorrect. Investment requirements pertain to rules and regulations regarding how institutions can invest their funds. This does not directly relate to the provision of security for accepting government deposits.

Choice D is incorrect. Operational requirements refer to the day-to-day functioning and management practices that an institution must adhere to, which does not necessarily include providing collateral as security for government deposits.