Solved: William the Conqueror’s marriage to which woman — the daughter of a powerful nobleman — helped William gain a strategic political ally on the European continent?

The Question: William the Conqueror’s marriage to which woman — the daughter of a powerful nobleman — helped William gain a strategic political ally on the European continent?

Margaret of Anjou
Eleanor of Aquitaine
Philippa of Hainault
Matilda of Flanders
Isabella of Valois

The correct answer is Matilda of Flanders.