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In a galvanometer, the magnetic torque experienced by the coil is balanced by the restoring torque of the spring. The magnetic torque (τ_magnetic) is given by τmagnetic = N I A B sin(α), where α is the angle between the magnetic field and the normal to the coil. For an ideal galvanometer, often a radial magnetic field is assumed such that sin(α) ≈ 1 or it is directly proportional to θ.The restoring torque (τspring) of the spring is given by τspring = kθ, where k is the spring constant and θ is the deflection angle.At equilibrium, τmagnetic = τspring. Therefore, N I A B = kθ (or Kθ = BIAN, using K for the spring constant as in the option).