What factor can significantly affect a horse’s racing performance?

The physical and mental condition of a horse is a crucial factor that can significantly affect its racing performance. A horse that is physically fit, healthy, and well-conditioned is more likely to perform optimally on race day. This includes aspects like muscle strength, endurance, and overall health. Moreover, the mental state of the horse plays an equally important role; a horse that is calm, focused, and confident is more likely to handle the stresses of racing better, leading to a more effective performance.

In contrast, factors such as the color of the horse do not influence performance. The geographic location of races can affect track conditions and climate but is secondary to the horse's condition. While the trainer's popularity might impact the horse’s public perception or the support it receives, it does not have a direct impact on the horse’s ability to race effectively on any given day.