Complete the tired student's complaint to their study group.

I __________ this essay since 8 AM, and I still haven't finished the introduction! My brain is officially on strike.

The correct answer is have been writing.

We use the Present Perfect Continuous to emphasize the ongoing, continuous nature of an action that started in the past and is not yet finished.

"Have written" (Present Perfect Simple) would imply the action is complete or focus on the quantity produced, which contradicts "haven't finished". "Am writing" (Present Continuous) cannot be used with "since 8 AM".