Which of the following is true?

The correct answer is Option (2) → Volt second (V s) is also an SI unit of magnetic flux.

Option (1) When the magnetic field lines are parallel to a surface, the magnetic flux linked with the surface is maximum. — Incorrect. The flux is \( \Phi_B=BA\cos\theta \), where \( \theta \) is the angle between \( \vec{B} \) and the normal to the surface. Lines parallel to the surface give \( \theta=90^\circ \) and zero flux; the flux is maximum when the lines are perpendicular to the surface.

Option (2) Volt second (V s) is also an SI unit of magnetic flux. — Correct. By Faraday’s law, \( \varepsilon=-d\Phi_B/dt \), so the unit of flux is the unit of emf times the unit of time, and \( 1\,\mathrm{Wb}=1\,\mathrm{V\,s} \).

Option (3) The less the time taken for the magnetic flux linked with a surface to change, the less is the induced emf. — Incorrect. The induced emf equals the rate of change of flux, so the same change of flux in less time gives a larger emf.

Option (4) The SI unit of magnetic flux is tesla. — Incorrect. Tesla is the unit of the magnetic field B. Flux is field times area, so its unit is the weber, \( 1\,\mathrm{Wb}=1\,\mathrm{T\,m^{2}} \).

NCERT (2026-27):
Magnetic Flux – “… The SI unit of magnetic flux is weber (Wb) or tesla meter squared (T m²). Magnetic flux is a scalar quantity.”

Faraday’s Law of Induction – “The magnitude of the induced emf in a circuit is equal to the time rate of change of magnetic flux through the circuit. Mathematically, the induced emf is given by”

    \( \displaystyle \varepsilon=-\frac{d\Phi_B}{dt} \qquad (6.3) \)