Final answer:
The correct answer is option D) A decree by the Oracle of Delphi.
Step-by-step explanation:
Athens had hegemony over the Delian League for several reasons. Firstly, Athens possessed a powerful navy, which was superior to that of other poleis in the league. This gave Athens control over the league, as they were responsible for providing and maintaining the ships.
Additionally, Athens employed persuasive diplomacy and negotiation skills to maintain their dominance, often stationing troops in garrisons in other cities and imposing new laws and taxes. Lastly, there was no unanimous vote by league members or decree by the Oracle of Delphi that granted Athens hegemony over the Delian League.