What Does the Underlying Assumption of Reasonable Assurance Regarding Implementation

What does the underlying assumption of reasonable assurance regarding implementation of internal control mean?

A) Auditor's are reasonably assured that fraud has not occurred in the period.
B) Auditor's are reasonably assured that employee carelessness can weaken an internal control structure.
C) Implementation of the control procedure should not have a significant adverse effect on efficiency or profitability.
D) Management assertions about control effectiveness should provide auditors with reasonable assurance.