Which of the following is a factor in the formation of a federation?

A factor in the formation of a federation is the "desire for a union". This implies that the desire of different regions or states to come together as one political entity with shared political and economic goals can lead to the formation of a federation. This can be driven by factors such as the need for stronger defense, economic integration, or the desire for greater political representation. The other options, such as absence of a common enemy, small population, and geographical incontiguity, are not necessarily factors that would lead to the formation of a federation.