Final answer:
The estimation method that involves functional experts making detailed estimates for work activities in the WBS is the Bottom-up estimation method.
Step-by-step explanation:
The method that requires estimates to be made at the detailed work activity level of the Work Breakdown Structure (WBS) by functional experts with knowledge of the required/available resources to complete the activity is a. Bottom-up estimation methods. The bottom-up approach involves breaking down a project into smaller parts, or work packages, and estimating the effort and resources needed for each part individually.
This method often results in more accurate and reliable estimates because it is based on the detailed knowledge of the people who are most familiar with the task. Bottom-up estimation contrasts with other estimation methods like the ratio method, apportion method, and parametric estimation which may rely more on historical data or statistical models rather than the detailed analysis of each task by a subject matter expert.