In order to find out which evidence will be the best one, we have to pay attention to the point on which author makes accent. As for me, author emphasizes that such physical activity as walking can positively affect our mental state, and the only option which maintains this idea is C. Endorphins released during walking build brain cells and make people feel good, because it points out the mood and state of a person.
You could come up with any independent clause and add it to this sentence.
For example, “The kids sprinted out the door”.
You can either add this to the front or end of the sentence.
“The kids sprinted out of the door as soon as the bell was rung”
“As soon as the bell was rung, the kids sprinted out of the door.”
Make sure to use a comma to separate the clauses if you’re going to put the main clause at the end of the sentence.
Hope this helped