Egyptian civilization was “the gift of the nile.” which explanation best supports this statement?

This was because of the Nile's yearly floods.

Explanation:

The development of the civilization of Ancient Egypt was only made possible by the flooding of the Nile. This great river was the only source of water that fertilized Egypt's otherwise desert land. A great monsoon brought rain which resulted in Nile's inundations. It was the most important yearly event back then and even represented the reference for the three seasons of Ancient Egypt: inundations, growth and harvest.

The floods occurred each year and the Egyptians still celebrate it as a national holiday starting in mid-august and lasting two weeks.