If R , L and C represent resistance, inductance and capacitance...

The time constant of an $RC$ circuit is given by $\tau = RC$, so the dimensional formula of $RC$ is $[\text{T}]$.

The time constant of an $LR$ circuit is given by $\tau = \dfrac{L}{R}$, so the dimensional formula of $\dfrac{L}{R}$ is $[\text{T}]$.

The resonant angular frequency of an $LC$ circuit is $\omega = \dfrac{1}{\sqrt{LC}}$. Since angular frequency has the dimensional formula $[\text{T}^{-1}]$, the term $\sqrt{LC}$ has the dimensional formula $[\text{T}]$.

The expression $\dfrac{R}{L}$ is the reciprocal of the time constant $\dfrac{L}{R}$, so its dimensional formula is $[\text{T}^{-1}]$, which represents frequency and not time.

Answer: $\dfrac{R}{L}$