Correct spelling, explanation: verb repay is irregular, so we cannot just add ed to it to create a past form. Past simple and past participle forms of repay is repaid, not repayed. Because this verb is irregular, repaid is the correct spelling.
Note: there is such word as repayed in nautical terms (past form of repay, meaning to close the leak).

Definition of repaid:
verb, past simple and past participle form of repay, to give back the money or payback
He repaid his debts a while ago.
The interest is repaid monthly.

Incorrect spelling, explanation: repayed is incorrect because the verb repay (just like pay) is irregular and we can just add ed ending to create past forms. Repaid is the correct form, just like paid is the past form of pay. Therefore repaid is the only correct option. Note: the word repay exists also in different meaning (nautical) and is regular, so its past form, is, in fact, repayed.