A is a yes, as nothing is blocking the view.
B and C are a no. Page 12 of the rules "If there is a model,
wall, or door in
any square between the Space Marine and his target then he cannot see it". The corner of the room is wall which blocks
LOS.
Had B been 3 squares closer to the room then I'd have said maybe, because that's the square that is the furthest away that either Marine can see without a corner getting in the way (taking a line through the centre of the squares would touch the corner). However, as shown on page 13, had the Marine been at that square and the stealers at X then he would not be able to see them due to the narrowing of the
LOS caused by not being in the doorway to the room. For simplicity sake, would it be safer to take the reverse of the Rooms
LOS on page 13 as being how looking out of rooms works too? If so, then reversing the models in that pic would have the Marines in the corners and not able to see a stealer next to the red X.
I think in 1st ed it would be a no too - I seem to remember that
LOS from centre of model square to centre of target square had to pass through clear squares only, and that even touching a corner was a no.