To improve the quality of a cost/capacity factor estimate, it is crucial to validate the capacity factor from similar, recently completed projects. This approach ensures that the estimate is based on real-world data and reflects current conditions and practices. Other factors, such as labor productivity, labor rates, and equipment costs, are also important but validating the capacity factor directly addresses the specific nature of the estimate being discussed. Therefore, the correct answer is A. Validate the capacity factor from similar, recently completed projects.