The correct answer is B) allowing parts of Egypt to elect their own rulers.
The action that would have completely changed ancient Egyptian civilization would have been "allowing parts of Egypt to elect their own rulers."
Let's remember that Pharaohs in ancient Egypt were named. People did not vote or had any opinion to choose their rulers. All the Pharaohs dynasties in ancient Egypt made citizens believed that they were the representation of the gods incarnated. That is why people feared and loved at the same time.
If ancient Egyptian people would have had the opportunity of having elections to vote for their government, history would have been very different.