Only two English kings have served with this name, one of which was the last King executed in England, occurring in 1649. Multiple French kings have served with this name, the last being the Tenth, who was deposed in the Revolution of 1830. A Habsburg King of this name served both as Holy Roman Emperor and King of Spain, when Martin Luther launched the Protestant Reformation. For 10 points, give this regnal name shared by multiple monarchs, and names the current heir to the British throne who was once married to Princess Diana.
a) Charles
b) William
c) Edward
d) James