Which of the following is used in lightning arresters?

In a virgin specimen of iron, the direction of spontaneous magnetisation varies from domain to domain.

An electron (charge -e) is rotating in a circular orbit of radius R, around a proton with charge +e at a constant angular velocity ω. The orbital magnetic dipole moment is

1 Bohr magneton = .

A particle of charge Q coulombs is bound elastically to an equilibrium position with a force constant of f newton per metre. The polarizability of the system is