According to Bohr's theory, the time averaged magnetic field at the centre (i.e nucleus) of a hydrogen atom due to the motion of electrons in the nth orbit is proportional to :(n=principal quantum number)

The magnetic field induced at the centre of an orbit due to an electron revolving in the nth orbit of hydrogen atom is given by
B=μ₀i/(2rₙ),
Where i is the current in the nth orbit,
rₙ the nth orbit and μ₀ the magnetic permeability of free space.
rₙ ∝ n²
i ∝ n⁻³
Therefore B ∝ n⁻⁴
So the answer is (d) n⁻⁴.