D. Rivals for the English Throne.
The Plantagenet Dynasty were split into two houses, York and Lancaster, both sides were descended from Edward III.
Long before this war, Henry Bollingbroke usurped Richard II and became Henry IV. Because of this, the crown technically passed from the York side of the family to the Lancaster side. Henry IV was succeeded by Heny V who survived a Yorkist plot, known as the Southampton Plot, to usurp him in favour of the Earl of March, and bring the crown back to the York side.
That failed and Henry VI succeeded Henry V as King of England, and due to circumstances in the 100 Years War, became King of France. But Henry VI ended up losing the 100 Years War, and his reign was marred by his mental instability and incompetence.
There were many elements behind the causes of the war, social changes, newly unemployed soldiers returning from France, lots of powerful nobles, such as Richard Neville (the Kingmaker), a weak and weakening king struggling to hold on to the loyalty of his nobles, with his competence in doubt, and the House of York remembering the fate of Richard II.
In the end neither of them won, when Henry Tudor, who was of Lancaster descent, defeated Richard III of the House of York, at Bosworth, and married Elizabeth of York to unite both the houses...and that ended the Medieval era in England.