Fill in the blank: 'discs must be within the following range: 1.25kg, 2.5kg, _____, 10kg, 15kg, 20kg and 25kg.'

The correct choice of 5kg makes sense within the context of the standardized weight increments used in powerlifting competitions. In the International Powerlifting Federation (IPF), weights on the bar are typically increased in specific increments to facilitate consistent weight training and competition standards.

The 5kg discs fit perfectly into the progression of weight increments between the lighter and heavier plates. Having a 5kg disc allows for a logical doubling of the 2.5kg and provides an effective mid-range option for lifters to make small and precise adjustments to their total lift weight.

Additionally, it keeps the series of available weights structured in a way that meets commonlifting protocols and norms, ensuring that athletes have access to a wide range of weights to suit their training and competitive needs. The other options do not align with the standard sizes commonly recognized in powerlifting, as they fall outside the typical disc increments established by the IPF.