Answer:
horse's
Step-by-step explanation:
"singular" means one. That means we can rule out "horses," which is more than one horse. Also there is no apostrophe so it cannot be possesive.
The next answer is "horses' " with an apostrophe '. The apostrophe makes it possessive, as in belonging to it, but there it is after the "s," meaning it belongs to more than one horse. Remember, we're looking for singular.
Finally, we have "horse's." Singular because there is one horse and possessive because the apostrophe is there before the "s." You could say "horse's saddle" and that means the saddle belongs to the one singular horse.
Hope this helped!