Comprehensive and Detailed Step-by-Step Explanation:

Determining downtime costs requires understanding the financial and operational impact on the business.

A. Fault tree analysis: Primarily used for identifying the root causes of failures, not for assessing downtime costs.

B. Cost-benefit analysis: Compares the costs and benefits of implementing controls but does not specifically address downtime.

C. Return on investment (ROI) analysis: Evaluates the financial return of an investment rather than the cost of an outage.

D. Business impact analysis (BIA): This is the BEST answer because a BIA identifies the effects of a disruption on critical business functions and quantifies the financial impact, including downtime costs.