When a superconductor is placed in a weak external magnetic field $H$, and cooled below its transition temperature:
A) The magnetic field is ejected
B) The magnetic field get magnetized
C) The magnetic field strength is increased
D) None of the above.

Hint: A magnetic field is a vector field, whereby a magnetic effect on electric loads, electric currents and magnetised materials are defined.

Complete step by step solution:
Meissner (or the Meissner – Ochsenfeld effect) is the expulsion from a superconductor of a magnetic field when coved below the critical temperature during its transition to the superconducting state.

The Meissner state is said to be a superconductor with little or no magnetic field within. When the magnetic field is too strong the Meissner state breaks down.

The magnet field is expelled when a superstructure is put in a weak external magnetic field $H$ and cooled under its transition temperature, and this phenomenon is called the Meissner effect. The potential of this magnetic field expulsion effect is determined by the existence of the balance created by the neutralisation of the superconductor unit cell.

Hence, the correct answer is option (A).

Note: Any ideal conductor stops the magnetic flux from changing due to the normal electromagnetic induction at zero resistance. However, the effect of Meissner differs from: when an ordinary conductor is cooled to move into a superconductor state, the magnetic flow is expelled while the transition is underway in the presence of a constantly applied magnetic field. This influence can be explained only by the London equation, not by an infinite conductivity. The placement and subsequent levitation of a magnet over a previously superconductive material does not reveal the Meissner effect, while a superconductor later repels an initially stationary magnet as it is cooled down to its critical temperature.