What is the dimensional formula of electrical mobility?

Electrical mobility ($\mu$) is defined as the magnitude of drift velocity ($v_d$) per unit electric field ($E$).

$\mu = \dfrac{v_d}{E}$

The dimensional formula of drift velocity $v_d$ is $[\text{LT}^{-1}]$.

Electric field $E = \dfrac{F}{q}$

The dimensional formula of force $F$ is $[\text{MLT}^{-2}]$ and that of charge $q$ is $[\text{AT}]$.

Thus, $[E] = \dfrac{[\text{MLT}^{-2}]}{[\text{AT}]} = [\text{MLT}^{-3}\text{A}^{-1}]$.

Therefore, the dimensional formula of electrical mobility is:

$[\mu] = \dfrac{[\text{LT}^{-1}]}{[\text{MLT}^{-3}\text{A}^{-1}]} = [\text{M}^{-1}\text{L}^{0}\text{T}^{2}\text{A}]$.

Answer: $\left[ \text{M}^{-1}\text{L}^{0}\text{T}^{2}\text{A} \right]$