The unit of rate constant for first order reaction is

$s^{-1}$

The correct answer is Option (4) → $s^{-1}$

For a first-order reaction, the rate law is:

$\text{Rate} = k$

Rate has units of mol L^-1 s^-1, and concentration [A] has units of mol L^-1.

Therefore, the unit of the rate constant k is:

$\frac{\text{mol L}^{-1}\text{s}^{-1}}{\text{mol L}^{-1}} = \text{s}^{-1}$