Which one of the following is dimensionless quantity?

Strain is defined as the ratio of change in dimension to original dimension (e.g., ΔL/L for longitudinal strain). Since it is a ratio of two quantities with the same units, strain is dimensionless. Stress (force per unit area), pressure (force per unit area), and force all have dimensions: stress and pressure have dimensions of [ML⁻¹T⁻²] (units of Pa or N/m²), and force has dimensions of [MLT⁻²] (units of Newton).