During the hours of darkness, the high or low beam is switched on. The high beam is switched to low beam in a lit populated area or to avoid blinding other drivers.
In darkness and poor visibility, a car cannot drive with only its clearance lights on: the high or low beam headlights must be on (clause 127). Both are allowed, and the choice depends on the situation: the high beam lights the road further ahead, but when it could blind other drivers, you must switch to the low beam. That is why the correct answer is not one type but both.
Option 1. The low beam is also allowed, and often required, depending on the situation.
Option 2. The high beam is also allowed when it does not blind other drivers.