If p is the resistivity of a material, and n is the number density of free electrons in it, the mean time between collisions between electrons in the material is given by: (m denotes the mass of an electron, and e denotes the charge on an electron)

Options

A. `(n e^2p)/m`
B. `(me^2)/(np)`
C. `( mp)/(n e^2)`
D. `m/(n e^2p)`

Explanation

To solve the problem, we need to derive the expression for the mean time between collisions (τ) in terms of resistivity (ρ), number density of free electrons (n), mass of an electron (m), and charge of an electron (e).

### Step-by-Step Solution:

1. **Understanding Drift Velocity (V_d)**:
The drift velocity (V_d) of electrons in a conductor can be expressed using the electric field (E) and the relaxation time (τ):
\[
V_d = \frac{eE \tau}{m}
...