We are given with following information about concentration of reactant with initial rate of reaction. Find out order of reaction with respect to that reactant.

Initital conncentration	Initial rate
0.005 M	7.5 x 10-4
0.02	3 x 10-3

The correct answer is 1.

To determine the order of the reaction with respect to the reactant, we can write the general rate equation as:

$\text{Rate}=k{\left[\text{Reactant}\right]}^n$

where $k$ is the rate constant and $n$ is the order of the reaction with respect to that reactant.

We are given two sets of measurements:
1. Initial concentration = 0.005 M, Initial rate = 7.5 x 10^-4
2. Initial concentration = 0.02 M, Initial rate = 3 x 10^-3

Let us calculate the factor by which the concentration increases:

$\text{Concentration increase factor}=\frac{0.02}{0.005}=4$

Next, let us calculate the factor by which the rate of the reaction increases:

$\text{Rate increase factor}=\frac{3\times {10}^{-3}}{7.5\times {10}^{-4}}=\frac{30\times {10}^{-4}}{7.5\times {10}^{-4}}=4$

Since the rate becomes 4 times when the concentration is increased by 4 times, we write the relation as:

$4^n=4$

This gives:

$n=1$

Thus, the order of the reaction with respect to the reactant is 1.