Final answer:
The conclusion about the first Thanksgiving that is best supported by Document 5 is that the Pilgrims held a feast and were joined by Native Americans, including Massasoit and about ninety other men.
Step-by-step explanation:
The conclusion about the first Thanksgiving that is best supported by Document 5 is that the Pilgrims held a feast and were joined by Native Americans, including Massasoit and about ninety other men. The Native Americans contributed to the feast by killing five deer, which were then roasted and eaten by those gathered at the Plymouth Plantation. This document provides evidence of the gathering, feasting, and the exchange of food between the Pilgrims and the Native Americans.