Correct!

What should you do when you're overtaking a horse and rider?

Sound your horn as a warning
Go past as quickly as possible
Flash your headlights as a warning
Go past slowly and carefully
Correct answer

DVSA theory test explanation

Horses can be startled by the sound of a car engine or the rush of air caused by a vehicle passing too closely. Keep well back and only pass when it's safe. Leave them plenty of room; you may have to use the other side of the road to go past safely.