Final answer:
The correct method used to stimulate urination in horses is gently rubbing dry hay ventral to the vulva in a circular manner.
Step-by-step explanation:
The correct method used to stimulate urination in horses is option d) Gently rubbing dry hay ventral to the vulva in a circular manner. This method stimulates the nerves in the area and can induce urination in horses. It is important to use a gentle touch and take into account the horse's comfort and well-being.
Taking the horse for a walk (option a) may encourage movement, but it does not directly stimulate urination. Rubbing a warm, wet cloth on the ventral abdomen (option b) may not have the same effect as the dry hay method. Occluding the nostrils (option c) is not a method used to stimulate urination in horses.