The diagram below shows horses in different periods of time on earth.
a- not always used their limbs for running.
b-not shown any evolutionary changes over the years.
c-evolved to them survive in changing earth conditions.
d- evolved to reduce their running and walking speeds.

Option C

Explanation:

There has been changes in the size of the horse over the period of time. They evolved in a way that they started running very fast.

Earlier the horses were dog sized and were not equipped for running, they usually used their limbs for walking.

Then during the course of time due to change in the earth condition they evolved to survive in the conditions.

Hence, option C is correct.