Which of the following is NOT true?

The correct answer is Option (4) → None of these

Option (1) Isolated magnetic poles (monopoles) are not known to exist. — True. A north pole or a south pole has never been found on its own. Breaking a magnet only gives smaller magnets, each with both poles.

Option (2) The net magnetic flux through any closed surface is zero. — True. This is Gauss’s law for magnetism. Every field line that enters a closed surface also leaves it.

Option (3) There are no sources or sinks of \( \vec{B} \). — True. Magnetic field lines form continuous closed loops. They do not begin or end at any point, because there are no magnetic charges.

Option (4) None of these — All three true. All three statements are true. They describe Gauss’s law for magnetism and its basis, the absence of magnetic monopoles. So none of them is untrue.

NCERT (2026-27):
Magnetism and Gauss’s Law – “The difference between the Gauss’s law of magnetism and that for electrostatics is a reflection of the fact that isolated magnetic poles (also called monopoles) are not known to exist. There are no sources or sinks of B; the simplest magnetic element is a dipole or a current loop. All magnetic phenomena can be explained in terms of an arrangement of dipoles and/or current loops.”

Magnetism and Gauss’s Law – “… Gauss’s law for magnetism is: The net magnetic flux through any closed surface is zero.”