In what direction can a horse rider go in the left-hand lane?

Answer: B — Any direction

Horse riders can go in any direction while riding in the left-hand lane. Option A is incorrect because they are not restricted to only going straight. Option C is incorrect because they can also turn right or go straight. Option D is incorrect because going in the right direction would require a lane change or riding on the opposite side of the road, which is not allowed. Therefore, option B is the best answer as it covers all possible directions a horse rider can go while remaining in the left-hand lane.