Answer B: The headmaster WANTS to speak to you.
Best explanation I found...
I look at this from an old English point of view. I dismiss that time dictates what is proper or not. As in old English, if you were found "wanting" it was used in a negative context, meaning you lacked a skill to do something you attempted. You would not say "I am wanting to learn to ...." whatever. You would say "I want to learn to ....". Yet you could be found wanting if you failed an attempt at something you did not have the skill for. Also for consideration is the premise of saying the most with the least. What takes less to say but conveys the most? "I want ..." or "I am wanting ..."? The use of “want” reduces confusion without using more than is needed to convey the message. Unfortunately as mentioned about the McDonald's ad, we've taught our children via marketing that incorrect use of English is ok. Most people have lost the skill or were never taught to speak English efficiently.