Answer:
My answers is Mediterranean Sea.
Explanation:
If I need to choose only one answer it would be Mediterranean sea, as it was to the north to Egypt, which meant that it was impossible to attack Egypt from North without navy.
Red Sea can be also the correct answer as it protects Egypt from the east, although Sinai Peninsula connects Egypt with Asia.
Tigris and Euphrates are in Mesopotamia, not Egypt.
Nile is not correct as the river flows across Egypt and couldn't prevent invasion.
