The MOST obvious security problem resulting from using a master-keying system is that

The correct answer is A. an unauthorized master key permits access through any lock in the series.

In PSP physical security, locks and key systems are part of structural security measures and access control countermeasures. The ASIS PSP Body of Knowledge includes locks under physical security countermeasure selection and implementation.

A master-keying system allows one master key to open multiple locks within a defined lock series. This is useful for management and emergency access, but it creates a major security risk: if the master key is lost, stolen, duplicated, or otherwise obtained by an unauthorized person, that person can access every lock controlled by that master key. Physical security lock guidance describes this as the most obvious security problem with master keys.

The other options are incorrect:

B is incorrect because master keying generally reduces the number of useful combinations, not increases them.

C relates more specifically to removable-core lock risks, not the main master-keying problem.

D is incorrect because close tolerances usually make picking harder, not easier.

Therefore, the correct answer is A.