It's A! Below your ribcage! (: It's just underneath. You may hear the phrase "sing from your diaphragm " they're just expressing that you should sing down deep! (: Technically, it's more so a phrase because if you attempt to sing from your diaphragm there's a bunch of muscle tightening, and definitely not comfortable!
I hope all is well, and you pass! (: Good luck, rockstar!