Answer:
no grass to for them to graze on
Explanation:
Given that Over many years the buffalo population out west gradually disappeared
We are given three statements and we have to find the correct likely reason for the above
change in climate over the years may not be most likely since climate changes gradually over the years, giving the animal time to adapt.
Decrease the number of humans in the west may not be correct as buffalo population has nothing to do with this
But grass is the main food for buffalo and hence no grass would make them starve and this may be the most likely reason.