What occurred before the soldiers were ordered to stop marching?

The reason "Until the DI told them to stop" is the correct choice lies in the sequence of events described in the statement. The soldiers were following a command structure, where the Drill Instructor (DI) holds authority over their actions. The use of "until" indicates that their marching was continuous and only ceased upon the DI's specific instruction. This establishes a clear cause-and-effect relationship, where the soldiers' actions are directly tied to the command given by the DI.

The other options do not establish the same direct consequence of stopping due to an order. While "Because it was too dark," "As they ran out of energy," and "After they sang a song" could provide reasons or contexts that might lead to stopping, they do not align with the authoritative command structure implied in the correct answer. The central idea is that the soldiers' actions are contingent upon the orders received, making the instruction from the DI the critical factor in determining when the marching stopped.