Which one of the following is dimensionless quantity?

Stress is defined as force per unit area. Its dimensional formula is $\dfrac{[MLT^{-2}]}{[L^2]} = [ML^{-1}T^{-2}]$.

Strain is defined as the ratio of change in dimension to the original dimension. Its dimensional formula is $\dfrac{[L]}{[L]} = [M^0L^0T^0]$. Thus, it is a dimensionless quantity.

Pressure is defined as force per unit area. Its dimensional formula is $\dfrac{[MLT^{-2}]}{[L^2]} = [ML^{-1}T^{-2}]$.

Force is defined as mass times acceleration. Its dimensional formula is $[MLT^{-2}]$.

Answer: Strain