In what order may the vehicles proceed if the sequence is 1, 2, and 3?

The correct order in which the vehicles may proceed is 1-2-3. This sequence indicates a clear priority or predetermined order for the vehicles based on factors such as road rules, traffic signals, or operational protocols that may be in effect.

For scenarios like this, the first vehicle, labeled as "1," typically has the highest priority to proceed, allowing it to move first. Following that, vehicle "2" can move next, indicating it likely has second priority. Finally, vehicle "3" follows, completing the sequence without any disruption.

Other sequences of movement present in the choices imply different prioritization or disorganization that does not align with established rules or logical flow, which might cause delays or confusion. For instance, in the scenario described, allowing vehicle "2" to go first, as in one of the other choices, could disrupt that established chain of command or priority. Therefore, maintaining the order of 1-2-3 ensures a systematic and efficient progression of movement.