In what order may the vehicles proceed?

The selected order, where vehicles proceed as 2-1-3, is correct because it typically adheres to traffic rules regarding right-of-way. In scenarios where vehicles approach an intersection or merge point, the general principle is that vehicles on the main road (often indicated by traffic signals or signage) have the right of way over vehicles that are coming from side streets or less busy roads.

In this case, if vehicle 2 is on a main road while vehicles 1 and 3 are on side roads waiting for the appropriate moment to merge or enter traffic, vehicle 2 would correctly be allowed to proceed first. Following that, vehicle 1, if it's yielding to vehicle 2's movement, would go next. Lastly, vehicle 3 would proceed after ensuring that both vehicles 2 and 1 have completed their movements safely.

Having an understanding of right-of-way rules is essential when determining the correct order of vehicles in various driving situations, ensuring safety and orderly flow of traffic.