Final answer:
The scenario requiring increased control testing by the auditor is when a high degree of reliance is placed on a particular control to limit substantive testing.
Step-by-step explanation:
The student has asked which scenario would require the auditor to increase the level of control testing for a particular control. The correct answer is a) A high degree of reliance is to be placed on the control to limit the amount of substantive testing required. This is because if an auditor is depending heavily on the effectiveness of a control to reduce the extent of substantive testing, they will need to thoroughly test that control to ensure it is operating effectively. Choice (b) notes a control over a less important risk, and (c) indicates a frequency issue, neither of which necessitate increased control testing by default. Choice (d) presents the presence of multiple controls, which doesn't inherently require increased testing for a single control.