Which of the following is NOT dimensionless?

Let us analyze the dimensional nature of each option:Specific resistance (Resistivity, $\rho$): It is defined from $R=\rho \frac{l}{A}$, which gives $\rho =\frac{R\cdot A}{l}$. Its unit is $\Omega \cdot \text{m}$ and its dimensional formula is $[{\text{M}}^1{\text{L}}^3{\text{T}}^{-3}{\text{A}}^{-2}]$. Thus, it is not dimensionless.Refractive index ($\mu$): It is the ratio of the speed of light in a vacuum to the speed of light in a medium ($\mu =\frac{c}{v}$). Being a ratio of similar quantities, it is dimensionless.Angle of friction ($\theta$): Angles are defined as $\frac{\text{arc}}{\text{radius}}$, which is a ratio of lengths, making it dimensionless.Specific gravity: It is the ratio of the density of a substance to the density of a reference substance (usually water). Being a ratio of identical quantities, it is dimensionless.