Which one of the following substances possesses the highest elasticity

The correct answer is Steel.

Explanation:
In physics, elasticity is defined as the property of a material by virtue of which it resists any change in its shape or size, and regains its original configuration when the external deforming force is removed.

The quantitative measure of elasticity is Young's modulus, which is denoted by the symbol $Y$. It is defined as the ratio of longitudinal stress to longitudinal strain:

$Y=\frac{\text{Stress}}{\text{Strain}}$

If we apply the same deforming force to identical wires made of steel and rubber, the steel wire undergoes a much smaller deformation (strain) compared to the rubber wire. Since strain is in the denominator, a smaller strain for the same applied stress results in a much larger Young's modulus.

Comparing the Young's modulus values ($Y$) for the given substances:

For Steel: $Y\approx 2\times {10}^{11}{\text{N/m}}^2$

For Copper: $Y\approx 1.1\times {10}^{11}{\text{N/m}}^2$

For Glass: $Y\approx 5\times {10}^{10}{\text{N/m}}^2$

For Rubber: $Y\approx {10}^6{\text{N/m}}^2$

Since steel has the highest value of Young's modulus, it is the most elastic substance among the options provided.