If a vehicle ahead is moving slowly and preparing to turn left, how may a following driver safely overtake it?

1. Overtake on the right, provided there is sufficient room on the carriageway without driving onto the sidewalk
2. Overtake on the left by crossing rapidly into the oncoming traffic lane
3. Flash headlights continuously until the leading vehicle aborts its left turn
4. Honk the horn and force past on the left shoulder