Which root word can stand alone?

There are multiple root words. I will give you three.

One of the meanings of "Form" is the shape of something.

Example: I am in the form of a dog.

"Act" is a movement or doing.

Example: We need to complete the second act of our play.

"Legal" relates to the law.

Example: That is legal.

Brainliest Appreciated.

i would say the correct answer is not concrete.