Review the item:
Mauricio would have _________ the seven-foot wave into shore if a huge shark had not been patrolling the waters underneath the surfboard, hungry for a dangling arm or leg.

The blank requires the past participle of ride, an irregular verb. Ridden is the past participle form.

Verb Simple
Present
Simple
Past
Past
Participle
ride ride(s) rode ridden

Rided, the form you chose, is a spelling error. Before you continue, review the rules for irregular verbs.

Try again to escape Grammar Hell!