b
\(x =\frac{1}{\sqrt{\mu_{0} \varepsilon_{0}}}=\operatorname{speed} \Rightarrow[ x ]=\left[ L ^{1} T ^{-1}\right]\)

\(y =\frac{ E }{ B }=\operatorname{speed} \Rightarrow[ y ]=\left[ L ^{1} T ^{-1}\right]\)

\(z=\frac{\ell}{R C}=\frac{\ell}{\tau} \Rightarrow[z]=\left[L^{1} T^{-1}\right]\)

So, \(x, y, z\) all have the same dimensions.