Thanks all for your replies.
It seems that most of the people agree to what Imbicatus suggested about stunned and staggered being totally different.
Also, your suggestion about downgrading Stunning Critical to Staggering Critical would make much sense and would be fair, but B seems the right answer according to RAW.
I also agree with Elicoor, the monster would be able to choose to fail its save voluntarily.
So I guess I should ask the bard/wizard about the monster being immune to stun, or just combine both critical feats with Critical Mastery to be safe.