C is correct because one of the principal advantages of computer-assisted audit techniques is their ability to analyze a complete transaction population rather than restricting the auditor to a manually selected sample.
Standard 14.1 states that data analysis software facilitates testing complete or targeted data populations, and the CIA Part 2 syllabus includes technology-enabled analytical approaches and data analysis methods within Information Gathering, Analysis, and Evaluation. Option A overstates the benefit because CAATs do not automatically make every audit faster; effectiveness depends on the data, tools, and procedures used. Option B may occur as a by-product of computerized work but is not the primary reason for using CAATs. Option D is also too specific and is not a general advantage of CAATs. Therefore, C most accurately describes their core audit value

When internal audit resources are limited, it is crucial to focus on the most critical aspects of the control environment. Preventive key controls are designed to prevent errors or irregularities from occurring, which are essential for maintaining a strong control environment. Given the mature control environment of the organization, prioritizing preventive key controls ensures that potential issues are addressed before they materialize, providing a proactive approach to risk management.
Reference: IIA Practice Guide "Assessing the Adequacy of Internal Controls"

The most important determinant of the objectives and scope of assurance engagements is the preliminary risk assessment performed by internal auditors. This assessment identifies the key risks that the engagement should address and ensures that the audit is focused on areas of highest risk and significance to the organization. While organizational charts, business objectives, and management requests are important inputs, the internal auditors' preliminary risk assessment ensures that the audit addresses the most critical areas.
References:
* The IIA's International Standards for the Professional Practice of Internal Auditing (Standards), specifically Standard 2210 - Engagement Objectives.
* The IIA's Practice Guide on Engagement Planning.