Text based evidence is basically evidence that is based off of a piece of text that you have read.
How far away they are from each other
The first choice is the best answer.
First, the Houyhnhnms are a rational species. Secondly, we know this when one friend visits the home where Gulliver is staying. She explains that she has arrived so late because her husband just died and she needed to make burial arrangements. Despite the fact that he literally just died, she behaves cheerfully.
Therefore, the Houyhnhnms view death as an event of life and they do not get upset when one occurs.
"Impact" is an effect.
So "impact" of the death means: the effect that death has on something.
For example, you could say:
The Impact of the death of the Emperor was the loss of respect for the country from the neighboring countries.
or you could say:
The impact of the death of her mother on her was devastating.