Let us find the dimensional formula of each physical parameter: (1) Energy density = $\frac{\text{Energy}}{\text{Volume}} = \frac{[M L^2 T^{-2}]}{[L^3]} = [M L^{-1} T^{-2}]$ (2) Refractive index = $\frac{\text{Speed of light in vacuum}}{\text{Speed of light in medium}}$. It is a ratio of identical quantities, so it is dimensionless $[M^0 L^0 T^0]$. (3) Dielectric constant is the ratio of permittivity of a medium to that of free space, so it is dimensionless $[M^0 L^0 T^0]$. (4) Young's modulus = $\frac{\text{Stress}}{\text{Strain}} = \frac{\text{Force/Area}}{\Delta L/L} = \frac{[M L T^{-2}]}{[L^2]} = [M L^{-1} T^{-2}]$ (5) Magnetic field $B = \frac{F}{qv} = \frac{[M L T^{-2}]}{[A T][L T^{-1}]} = [M T^{-2} A^{-1}]$

Comparing the dimensions, we can see that energy density (1) and Young's modulus (4) have the same dimensions of $[M L^{-1} T^{-2}]$. Refractive index (2) and dielectric constant (3) also have the same dimensions, but '2 and 3' is not an option.