Take another look at the sentence:
My grandmother doesn't trust banks, so she has __________ money all over the house. Once, I pulled an old Reader's Digest off a shelf and found seven $100 bills stuck between the pages.
The sentence requires the past participle of hide, an irregular verb. Hidden is the past participle.
|Verb||Simple Present||Simple Past||Past Participle|
Hiden, the form you chose, is a spelling error. Before you continue, review the rules for irregular verbs.