Which theme in Hamlet is reinforced by the scene with the gravediggers?

moral corruption

mortality

revenge

appearances versus reality?

The correct answer is mortality.

During the scene with the gravediggers, Hamlet is reminiscing about Yorick, the court jester, while holding his skull. He says that life is fleeting, and whoever we are, we must die in the end. He mentions great historical people such as Caesar and Alexander the Great, who turned to dust in the end. No matter how powerful they were, death still waited for them.

I'd say the answer is "mortality". At the end of the scene, Hamlet gives a speech regarding the circle of life, and how we live, but ultimately return to dust after we die.