The mobility of charge carriers increases with

increase in the average collision time

The correct answer is Option (4) → increase in the average collision time

Mobility is defined as:

$\mu = \frac{v_d}{E} = \frac{q \tau}{m}$

where $q$ = charge, $\tau$ = mean collision time, $m$ = mass of charge carrier.

Thus, mobility increases when $\tau$ (average collision time) increases.

Correct option: increase in the average collision time