In which year did English barons force King John to agree to Magna Carta?

Explanation

The barons acted after a period of heavy taxation and disputed government, and the agreement they extracted bound the king to observe named customs, so it was the outcome of armed pressure rather than a gift from the crown. The year of the Norman Conquest fails because that event lies a century and a half earlier and settled who would rule England, not what limits applied to a king already reigning.