Answer:
First Amendment, freedom of speech
Step-by-step explanation:
Freedom of speech is a constitutional right guaranteed in the U.S. Constitution in the First Amendment. Sometimes it is difficult to determine what this freedom means and how far it expands.
It is the freedom of expression and it includes any medium: orally, in written, in print, on the internet or in any art form.
As Harry wanted to write something on a private sign, he had the constitutional right to do so and the court decision was justified. He could have been denied this right if the sign was close to an educational center, in a proximity of a road sign, or the writing would have been considered a threat to public safety.