Answer:
Ridden.
Have you ridden on the hay wagon?
Explanation:
The correct way to write the above mentioned sentence will be
"Have you ridden on the hay wagon?
The past and past participle of ride is rode and ridden. And when the sentence have a 'have/has' which are auxiliary verbs, or when the sentence have a 'be' form of the verb, only then do we use the past participle form of the verb.
