A cat lies on the floor? can you say that no force acts on the cat? or is it correct to say that no net force?
An object has no acceleration if there is no net force on the object.
The cat is not moving so there is no acceleration on the cat and therefore no net forces.
I am assuming the cat is on the earth, and so there is the force of gravity on the cat, and an equal and opposite force on the cat from the floor the cat is laying on. So it would be incorrect to say there were no external forces on the cat.
I hope that helps, the correct answer is no net force on the cat.