During the collision a relatively large force acts on each colliding particle for a relatively –––– time

A relatively little period of time is spent in contact between the colliding items. The colliding bodies experience deformation at the site of contact during collision in a very brief period of time, and they quickly return to their original shapes and sizes (totally for elastic deformation and partially for inelastic deformation).

From the question,

$$\begin{gathered} ∆{\mathrm{P}}_{21}=-∆{\mathrm{P}}_{12} \\ ∆\mathrm{P}=\mathrm{F}∆\mathrm{t} \end{gathered}$$

The large force acts during a very short period.

Hence the correct answer is short.