The correct answer is: "First Amendment"
According to the First Amendment to the US Constitution, the US Congress cannot have the power to confer privileges to any religion over others in any piece of legislation issued by this chamber, for example, by positioning the most widely followed cult in the country as an official one, or by granting certain advantages to the institutions or communities related to a specific religion.
This is the reason why praying cannot be accepted in public schools, as it would position a certain cult over the others.