I adopted a cat on a whim. I'm taking her back today.
What was your reason for adopting a cat?
I was feeling lonely since my fiance was not around and I wanted a companion.
Adopting a pet should be thought of as adopting a family member, it shouldn't be done on a whim. Bringing the cat back to the shelter you adopted it from is the responsible thing to do.
I'm going to leave the cat at the front door and run away.
Adopting a cat on a whim is not a great thing to do, and leaving the cat out by itself after you decide you do not want it anymore is even worse. If you truly do not want to take care of the cat, the only action that needs to be done is for you to take the cat back to where you purchased him/her from.