a
(a) According to Bohr's hypothesis, electron can revolve only in those orbits in which its angular momentum is an integral multiple of \(\frac{h}{2 \pi},\) where \(h\) bemg the Planck's constant.

In these orbits, angular momentum of electron can have magnitude as 

\(\frac{h}{2 \pi^{\prime}} \frac{2 h}{2 \pi^{\prime}} \frac{3 h}{2 \pi} \cdots \cdots \cdot\) etc \(,\) but never as \(\frac{1.5 h}{2 \pi^{\prime}}, \frac{2.5 h}{2 \pi^{\prime}}, \frac{3.5 h}{2 \pi} . \ldots . . .\) etc.

This is called the quantisation of angular momentum.