Read the Passage carefully and answer the Questions.

Kinetic studies not only help us to determine the rate of a chemical reaction but also describe the conditions by which the reaction rates can be altered. The factors such as concentration, temperature, pressure and catalyst affect the rate of a reaction. The speed of a reaction or the rate of a reaction can be defined as the change in concentration of a reactant or product in unit time.

Rate = $k [A]^x [B]^y$

Here k is the rate constant

$x + y$ gives the overall order of a reaction, whereas x and y represent the order with respect to the reactants A and B, respectively. 

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:

$\text{Rate} = k[A]$

Rate has units of concentration per time $(\text{mol L}^{-1}\text{s}^{-1})$.

So, the unit of $k$ is:

$k = \frac{\text{Rate}}{[A]} = \frac{\text{mol L}^{-1}\text{s}^{-1}}{\text{mol L}^{-1}} = \text{s}^{-1}$