Primary Focus: Functions

D.

When developing a risk-based IS audit plan, the primary focus should be on functions that are considered critical to business operations. This means that the audit plan should prioritize the identification and assessment of risks that could impact the organization's ability to achieve its objectives and maintain its operations.

Option A, "considered important by IT management," is not necessarily the best approach because IT management may not have a full understanding of the business's critical operations and risks. They may prioritize certain IT functions based on their own preferences or biases, rather than the organization's strategic objectives.

Option B, "with the most ineffective controls," is also not the best approach because it does not necessarily identify the most critical risks to the organization. While it is important to address ineffective controls, the focus should be on risks that could have the greatest impact on the organization's ability to achieve its objectives.

Option C, "with the greatest number of threats," may be a consideration, but it is not the primary focus. The focus should be on identifying and assessing the risks that could have the greatest impact on the organization's ability to achieve its objectives, regardless of the number of threats.

In summary, the primary focus of a risk-based IS audit plan should be on functions that are considered critical to business operations, as this approach prioritizes the identification and assessment of risks that could have the greatest impact on the organization's ability to achieve its objectives and maintain its operations.