Choice "C" is correct. If analytical procedures suggest unexpected relationships, the auditor would perform additional tests of details of the accounts involved.
Choice "b" is incorrect. The identification of unexpected relationships as a result of analytical procedures does not necessarily mean that irregularities exist in the relevant account balances, although this is a possible explanation.
Choice "d" is incorrect. The identification of unexpected relationships as a result of analytical procedures does not necessarily mean that internal control activities are not operating effectively, although this is a possible explanation.
Choice "a" is incorrect. The identification of unexpected relationships as a result of analytical procedures does not necessarily mean that communication with those charged with governance should be revised, although this is a possible consequence.