Correct!

What should you do if you're riding at night and are dazzled by the headlights of an approaching vehicle?

Switch off your headlights
Switch to main beam
Slow down and stop if necessary
Correct answer
Flash your headlights

DVSA theory test explanation

If your view of the road ahead is restricted because you're being dazzled by approaching headlights, slow down and, if you need to, find a safe place to stop until your vision is restored.