Answer:
Our Constitution is based on certain principles .
Step-by-step explanation:
The term principle is a noun and it indicates fundamental beliefs that something is based on, whether it be a constitution in this case, or somebody's beliefs. Scientific principles are the laws of science.
The correct use of the word in other sentences would be:
Our experiment was based on scientific principles.
The principles of my parents have always stuck with me.
On the other hand, principal is an adjective and it refers to something that is primary, main. It can also be used as a noun: The principal visited our class today. In this case, it refers to the headmaster of a school.