King John was primarily forced to sign the Magna Carta because he had raised taxes and seized the lands of nobles, which angered them and led to widespread discontent. This resulted in a rebellion by the barons, pushing King John to negotiate and ultimately sign the Magna Carta in 1215 as a means to appease them and restore order.
So, the best response would be: He had seized the lands of nobles. Conversely, this is closely tied with the raising of taxes. If you can only choose one, go with the land seizure.