\(B=\frac {μ_0NI}{2R}\)
\(\frac {B_x}{B_y}=\frac {N_xR_y}{N_yR_x}\)
\(\frac {B_x}{B_y}=\frac {200×20}{400×20}\)
\(\frac {B_x}{B_y}=\frac 12\)
So, the correct option is (B): \(1:2\)

The magnetic field is a field created by moving electric charges. It is a force field that exerts a force on materials such as iron when they are placed in its vicinity. Magnetic fields do not require a medium to propagate; they can even propagate in a vacuum. Magnetic field also referred to as a vector field, describes the magnetic influence on moving electric charges, magnetic materials, and electric currents.