What geographic feature made possible the union off upper and lower egypt?

The correct answer is - The Nile River.

The Nile River made it possible that the upper and lower Egyptian kingdoms unite. The Nile was and still is of extreme importance to Egypt. It was the the river that was giving life, the river that was allowing travel, transportation, and communication with the distant parts. The Nile River was the connecting piece between the two parts of Egypt, and the unification served both kingdoms well as it made them much stronger in every field.