When molecules collide with the walls of their container, is exerted

pressure

stress

gravity

friction

I believe your answer would be friction ...

Answer: pressure.

Justification:

1) Pressure is the force exerted by area unit: P = F / S

2) Then, any force applied on a surface exerts pressure.

3) When particles (molecules) collide with the walls of their container, they exert force on the walls, ergo pressure is exerted.

4) The kinetic molecular theory explains the increase of the pressure of the gases resultant of the compression by the fact that the particles are confined to a smaller volumen and so more particles will collide with the walls of the container.