Let's assume that when we provide service against cash then accounting entry will be

Cash Account Debit ( As cash received to company)

To Service Revenue Account Credit (Service provided to customer)

So when we received cash but service not provided then accounting entry will be

Cash Account Debit ( As cash received to company)

To Unearned Revenue Account Credit (Service not provided to customer)

Hence when we provide service to customer for which cash already received we need to reverse the Unearned revenue account balance and post to service revenue account.

So Correct Journal entry is

Unearned Revenue Account Debit 9000

To Service Revenue Account Credit 9000

As advance payment is 9000 hence all amount will be transfer to service revenue.

Hope given answer will solve your query.