The resilience requirements of an application are best determined by a business impact analysis (BIA). A BIA identifies the critical functions of an application and the potential impact of their loss, which helps in understanding the level of resilience needed. This process focuses on the effects of disruption and helps in establishing priorities for recovery, making it the most suitable method to determine resilience requirements. Other methods like cost-benefit analysis, threat assessment, or risk assessment might provide valuable insights, but they do not focus primarily on the impact of disruption and business continuity as a BIA does.