A set of principles by which a country is governed, including its institutions and how their power is kept in check, is called a
Story
A story is a fictional or factual narrative; it does not define the legal rules governing a state or the rights of citizens.
The Law
Laws are individual rules passed by Parliament; a constitution is the overarching framework setting out how laws are made and citizens' rights.
History
History is the record of past events, not a legal document governing how laws are made.
Constitution
A constitution is the set of fundamental rules governing how a country is run, how laws are made, and the rights and duties of its citizens.
Correct answer
Explanation
A constitution is a set of principles by which a country is governed. The British constitution is not written down in any single document, so it is described as unwritten.