None of you are correct, The B757 is classified as a "H" (heavy) for departure ahead of another aircraft and for wake turbulence intrail spacing in flight, and is categorised as a "M" (medium) when it is following preceding traffic, either on take off or in flight spacing.
So, a "H" departure (ie an A330) with a B757 to follow, then the 757 is considered to be an "M". If the B757 is ahead of a B737 (medium) then a wake turbulence spacing appropriate to a "H" leading is required.
I'm a Tower controller in Hong Kong.