The correct answer is 3) Thrust decreases

Since thrust = massflow ( uexit - uo). When the aircraft starts the takeoff roll uo=0 and at the end of the takeoff roll uo may be an appreciable fraction of uexit. Physically, what is happening is that relative to the engine the molecules coming in have ever increasing momentum and this acts counter to the momentum that the engine gives the molecules (the exit flow). In reference to the PRS question about "rock-breathing" propulsion, it is like having the person on the dock throw the rocks with greater and greater velocity (relative to the boat).

Class performance (2004):

Class performance (2003):

Class performance (2001):