Ordinary traffic signals use only three colours — red to stop, amber to caution and green to go — to control the general flow of vehicles at an intersection.

The governing idea is that a movement instruction reserved for emergency vehicles must use a colour set apart from those three ordinary phases, so it is not confused with normal traffic commands.

A flashing blue signal is the colour associated with emergency vehicles, marking their special right of movement through the intersection.

A flashing red signal means treat the junction like a stop sign — a general warning to halt — so it is not reserved for emergency movement.

A flashing yellow signal tells all drivers to slow and proceed with caution, again a general instruction rather than an emergency-vehicle one.

A flashing green is not a standard emergency reservation either; green simply conveys the ordinary permission to proceed.

Because blue is the colour set aside for responders' special movement, it is the correct signal.

Hence, the answer is A flashing blue signal is used for emergency vehicles.