Final answer:
The adjusting journal entry required on December 31 is to credit revenue $400, as only a portion of the revenue has been earned by that date.
Step-by-step explanation:
The adjusting journal entry required on December 31 is to credit revenue $400. This is because $1,200 of revenue was received in advance for work to be performed over 12 months, and the transaction was already recorded in an income statement account. December 31 represents 4 months of the 12-month period, so the adjustment is made to recognize the portion of the revenue that has been earned.