Final answer:
To avoid frightening a horse when approaching, one should slow down and provide ample room while passing, ensuring the safety of all parties.
Step-by-step explanation:
When approaching someone riding a horse, it is important to avoid frightening the animal to prevent accidents. The correct action to take is to slow down and allow plenty of room when passing. Making loud noises such as sounding your horn, speeding up, or ignoring the rider can startle the horse and lead to dangerous situations. Instead, approach with caution and provide the horse and rider enough space to ensure safety for everyone involved.