What motivates me the most is if someone is counting on me.
That is probably the one thing that motivates me the most as well.
However, I must say that if someone tells me I can't do something, a combination of rage and determination rises up within me. Unless it's something I know I'll never do or master.
If someone tells me that they believe in my abilities, I just swell with pride. No amount of praise will motivate me, it's only when my abilities are questioned that I motivate myself.
I recall someone did a study in my school on our rowing team, and found that people who were receiving negative feedback (i.e. choice A) did better on their 2,000 meter row than those who were encouraged (i.e. B).
This obviously only applies to certain people in certain situations, but food for thought