Final answer:
In the context of auditing, a material weakness is considered when there is a reasonable possibility of a material misstatement due to a control deficiency.
Step-by-step explanation:
In the context of auditing, a control deficiency is a weakness in the design or operation of internal controls that could affect the organization's ability to prevent or detect material misstatements in its financial statements. A control deficiency is considered a material weakness if it is reasonably possible that a material misstatement could occur and not be prevented or detected on a timely basis.
Therefore, the level of probability required for a control deficiency to be considered a material weakness is 'reasonable possibility'. This means that there is a likelihood, although not certain, that a material misstatement could occur and go undetected.
Option C, 'reasonable possibility', is the correct answer.