hiking/ trekking

I have the sentence: ‘…through the mountains on horse back was an amazing experience.’
As options I have: a. hiking b. wandering c. trekking d. marching
c. trekking is given as the correct option, why c.? Shouldn’t it be a. hiking the correct answer?
Thank you for your time!

Hiking - travelling on foot; walking.
Trekking - a long walk or ride (on horse, pony, donkey etc.), particularly through mountains. Also, ‘pony -trekking’ is the activity of riding ponies in the countryside for pleasure.