The Middle Kingdom began with the reunification of the country under which leader at the beginning of the eleventh dynasty?

Mentuhotep I?

You're right, Yafa! Thanks for your answer.

The reason I put a question mark is because the website I looked at said the fourth dynasty, not the 11th.

There seems to be many variations of his name, whether he was the first or second, and time, depending upon your source. Mentuhotep I (or II) was in the eleventh dynasty at the formation of Middle Kingdom Egypt.

Article - See second paragraph.