1215.
Yes, but that Magna Carta only remained a legal document for three months, when the Pope Innocence III denounced it, making it null and void, as it had been signed by King John under duress.
It was the 1225 version that came into law, signed by boy King Henry III in return for tax concession. He re-issued a shortened version when he reached adulthood in 1227, and it was parts of that which was the first to be recognised in English law. However, it is the 1297 version, signed by King Edward I that has parts still recognised by current English Law.
Therefore, if the question asked during
Who Wants to be a Millionaire was simply "who signed the Magna Carta?", and not "What King signed the
first Magna Carta in 1215?", then various answers would be valid!