Electrically charged particles from space travelling at speed of several hundred km/sec can severely harm living beings if they reach the surface of the earth. What prevents them from reaching the surface of the Earth?

The correct answer is A. Charged particles from the solar wind and cosmic rays are deflected by Earth's magnetic field. A moving charge experiences a Lorentz force perpendicular to both its velocity and the field, so the particles spiral along the field lines. The lines converge near the poles, which is why many particles are funnelled there and produce aurorae.

• Option B is wrong: the ozone layer absorbs harmful ultraviolet radiation. It does not reflect charged particles back into space.
• Option C is wrong: moisture plays no role in stopping charged particles. Water vapour is concentrated in the troposphere, far below the region where these particles are first stopped.
• Option D is wrong because A is correct.

Core concept: the geomagnetic shield, generated by convection of molten iron in the outer core (the geodynamo). Mains connection: space weather, with solar storms disrupting satellites, GPS and power grids, and the protective role of the Van Allen belts. The field is weaker over the South Atlantic Anomaly and the magnetic poles drift, but the principle holds.

Trap: choosing B because ozone is the first protective layer that comes to mind. Ozone protects against UV radiation, not charged particles.