Which is quantised in Bohr's atomic model?

The correct answer is Option (4) → angular momentum of the electron

Bohr's second postulate (the quantisation condition) says that the electron revolves only in those orbits for which its angular momentum is an integral multiple of \( h/2\pi \):

    \( \displaystyle L = mvr = \frac{nh}{2\pi}, \quad n = 1, 2, 3, \ldots \)

This is the quantity the model quantises by postulate. The radius, speed and energy of each allowed orbit are then worked out from this condition together with the Coulomb force, so the energy levels \( E_n = -13.6/n^{2} \) eV come out as a consequence of quantising the angular momentum.

NCERT (2026-27):
Bohr Model of the Hydrogen Atom – “… (ii) Bohr’s second postulate defines these stable orbits. This postulate states that the electron revolves around the nucleus only in those orbits for which the angular momentum is some integral multiple of h/2π where h is the Planck’s constant (= 6.6 × 10^–34 J s). Thus the angular momentum (L) of the orbiting electron is quantised.”