The correct answer is C. Critical region.
A critical region is a section of code that may only be executed by one process at any one time. This is to ensure that the data in the critical region is not corrupted.
A critical resource is a resource that is shared by multiple processes. This resource must be managed carefully to ensure that it is not accessed by multiple processes at the same time.
CPM is a project management technique. It stands for Critical Path Method.
Gray code is a type of code that is used to represent numbers. It is a non-repeating code.
None of the above is a section of code that may only be executed by one process at any one time.