I was thinking

Unearned Fees XXXX
Fees Earned XXXXX

Is this correct?

Fees billed but unearned? Are you billing in advance for work that is not done yet?

Anything that is "billed" is accounts receivable. Anything not yet earned is a liability, unearned revenue.

But your entry is indicating the adjusting entry when the money was paid in advance, but is now earned. This moves it out of the unearned account and into the earned account. (There is no billing in your entry.)

Because I've never seen the account Fees Earned in life ever, and only in textbooks, I'm going to jump to the conclusion that this is homework. And if it's homework, I've never seen a textbook do "fees billed but unearned." Are you sure that's what it says?