The Magna Carta was a document created to prevent rebellion in England and limited the power of the king or queen.
The Magna Carta was seen as a cornerstone document because it limited the power of the king. Measures such as habeaus corpus were put into place to ensure people were not arrested or imprisoned without rights to know their charges and to have a judgement on their case. The ideas of the document became a foundation for modern English government and the American Constititution/Bill of Rights.
The correct answer is the Magna Carta. The cornerstone of English democracy is the document known as the Magna Carta. It is considered as a document constituting a fundamental guarantee of rights and privileges.