Final answer:
In a collision where two boxes on a frictionless surface stick together, momentum is conserved. Kinetic energy is not conserved due to it being an inelastic collision. Potential and angular momentum conservation are not applicable in this scenario.
Step-by-step explanation:
If two boxes on an icy, frictionless, horizontal surface collide and stick together, the quantity conserved is momentum. In physics, when dealing with collisions, especially inelastic ones where objects stick together, it's a fundamental principle that momentum is always conserved in the absence of external forces. Although the kinetic energy in inelastic collisions typically changes (usually decreases because energy is lost to sound, heat, deformation, etc.), the combined momentum before and after the collision remains constant. Potential energy is generally conserved in any process where there is no change in the gravitational or elastic potential energy. Lastly, angular momentum is conserved in scenarios where no external torques are acting on the system, which is not applicable in this context because we're dealing with straight-line motion, not rotation.