Answer:

Step-by-step explanation:
Energy density in magnetic field is given as;

where;
B is the magnetic field strength
Energy density of electric field

where;
E is electric field strength
Take the ratio of the two fields energy density


But, Electric field potential, V = E x L = IR (I is current and R is resistance)

Now replace E x L with IR

Also, B = μ₀I / 2πr, substitute this value in the above equation

cancel out the current "I" and factor out μ₀

Finally, the equation becomes;

Therefore, the correct option is (d) μ₀/ϵ₀ (L /R 2πr)²