Answer: Unilateral contract
Explanation: A Unilateral contract is a form of contract where a promise is made by one party to another, this contract is normally on a condition that the receiver of the promise in the contract would complete some task(s), in order to receive the promise.
Mark made a promise to his staffs in the newspaper newsroom to be fulfilled, if the task was accomplished by anyone. Of which Anna completed the task and claimed the promise by the editor.