Question

during collision what remains constant

Answer 1

If you are asking this in physics or chemistry, then mass and energy always remain constant, according to laws.
Hope this will be helpful :)

Answer 2

Answer:

During collision, which may be either elastic or inelastic, the momentum of individual objects may change in a system, but it is the "total momentum" of all the objects together that always remain constant in an "isolated system".

Explanation:

The "kinetic energy" is not constant during collision, although the "kinetic energy" before the collision is seen to be equal to the kinetic energy after the collision. This can thus be stated as the "Law of Conservation of Momentum".