If Queen Elizabeth II dies and Charles, Prince of Whales becomes king, Who will become the Heir apparent?

When Charles Prince of WALES becomes King the next in line is Prince William because he is the first born child of Prince Charles. Princess Margaret remained a Princess until she died, a Countess is the lower title. I think you are confused with Royalty titles. To the person that said I was wrong regarding Princess Margaret not being the Countess of Snowdon, just where in my answer did I say that?