What Is the Definition of a Critical Function

What is the definition of a critical function?

A) A business function that is so critical, it cannot be disrupted for more than a couple hours without severe business impacts
B) A business function or process that cannot be disabled for more than a couple business days, without it having a negative impact on the organization
C) A function or role within the organization which is irreplaceable
D) A function that is managed by the Crisis Management Team