When does the blocking creature choose the damage assignment order?

The correct answer is that the blocking creature chooses the damage assignment order during the declare blockers step.

In this phase of the combat procedure, players declare which creatures are blocking which attacking creatures. Once blockers are declared, the player controlling the blocking creature is given the opportunity to assign damage order among the blocking creatures if there is more than one that is blocking a single attacker. The damage assignment is important, especially in cases where the attacking creature has trample or if the blocking creatures have different toughness levels.

Proper assignment of damage can be crucial to strategies during combat. For example, if an attacking creature with trample is blocked by two creatures, the controlling player must decide how much damage to assign to each blocker and how much (if any) can be assigned as excess damage to the defending player. This decision-making happens immediately after the blockers are declared, thus occurring in the declare blockers step.

The other stages listed do not provide for damage assignment; thus they are not relevant in this context.