On September 1, the company would debit Prepaid Rent for $27,000 and credit Cash for $27,000. By December 31, four months have passed, so the company has used up $9,000 of the prepaid rent ($2,250/month * 4 months).
The adjusting entry on December 31 would be to debit Rent Expense for $9,000 and credit Prepaid Rent for $9,000. This recognizes the rent expense for the period and reduces the prepaid rent asset by the amount used.