The modulus of elasticity (E) is a measure of the stiffness or rigidity of a material. It describes the ability of a material to deform under applied stress and then return to its original shape when the stress is removed. In the case of steel used in reinforcement, the modulus of elasticity refers to the Young's modulus.

Young's modulus is defined as the ratio of stress to strain within the elastic limit of a material. It is commonly denoted by the symbol E and has units of stress (N/mm^2 or MPa) since stress is force per unit area.

Now, let's analyze the given options:

a) 2 x 10^5 N/mm^2
b) 3 x 10^5 N/mm^2
c) 0.02 x 10^5 N/mm^2
d) 0.2 x 10^5 N/mm^2

To determine the correct answer, we need to consider the typical range of Young's modulus for steel. Steel is a strong and rigid material, so its Young's modulus is relatively high.

The typical range for the modulus of elasticity of steel is between 190 GPa and 210 GPa, which is equivalent to 190 x 10^3 N/mm^2 and 210 x 10^3 N/mm^2, respectively.

Comparing these values with the given options, we can see that option a) 2 x 10^5 N/mm^2 falls within the typical range for steel's modulus of elasticity. This makes it the correct answer.

In contrast, option b) 3 x 10^5 N/mm^2 is outside the typical range for steel and is too high. Options c) 0.02 x 10^5 N/mm^2 and d) 0.2 x 10^5 N/mm^2 are both too low and not reflective of the actual modulus of elasticity of steel.

Therefore, the correct answer is option a) 2 x 10^5 N/mm^2.