Defining activities by business processes is a structured approach that allows the internal audit activity to identify engagement opportunities effectively. This method ensures that all critical processes are reviewed systematically and that risks are identified and assessed in the context of how they affect the entire business process. This approach is comprehensive and aligns with best practices in internal auditing.
References:
The Institute of Internal Auditors (IIA) Standards
Internal Audit Planning and Engagement Standards

The most appropriate approach for internal audit activity to follow up on management action plans is to create a tracking system. This ensures that follow-up activities are systematically monitored and documented. Such a system can track the status of action plans, provide reminders for due dates, and record progress updates, thus ensuring that management's corrective actions are implemented and effective. Regular monitoring and tracking are essential to verify that issues identified in audits are addressed in a timely manner.
Institute of Internal Auditors (IIA) Standards: Implementation Standards 2500 - Monitoring Progress COSO Framework: Monitoring Activities Component

Stratified sampling is a technique that involves dividing a population into subgroups (strata) that share similar characteristics and then taking a sample from each subgroup. In the context of testing purchase transactions by different procurement officers, the transactions can be stratified based on the officers, ensuring that the audit team selects a sample that represents each officer's transactions. This method helps in achieving a more accurate and reliable assessment of the procurement process across all officers.
The Institute of Internal Auditors (IIA), Practice Guide on Sampling
"Auditing and Assurance Services: An Integrated Approach" by Alvin A. Arens, Randal J. Elder, Mark S.
Beasley

When employees continue to violate segregation-of-duty controls despite previous recommendations, the most effective approach is to recommend appropriate awareness training. This training can help employees understand the importance of these controls and how to comply with them, addressing the root cause of the violations. References: = IIA Standard 2130 - Control and IIA Practice Guide: "Auditing Segregation of Duties".

In the "Define IT Universe" stage of the IT audit plan development process, the primary action is to identify significant applications that support the business operations. This step involves mapping out the IT environment, including key systems, applications, and infrastructure components that are critical to achieving business objectives. By identifying these significant applications, auditors can focus on areas that have the greatest impact on the organization's performance and risk profile. This stage sets the groundwork for subsequent steps such as risk assessment, ranking subjects, and selecting audit engagements.
IIA Global Technology Audit Guide (GTAG): "Developing the IT Audit Plan" ISACA's COBIT Framework for IT Governance and Management