\({V_{{\text{treat }}}} = \frac{{3\,{\text{cm}}}}{{0.2\,{\text{ns}}}} = \) \(\frac{{3 \times {{10}^{ - 2}}{\text{m}}}}{{0.2 \times {{10}^{ - 9}}{\text{s}}}} = 1.5\,{\text{m}}/{\text{s}}\)

Refractive index of the medium

\(\mu  = \frac{{{V_{{\text{air }}}}}}{{{V_{{\text{med }}}}}} = \) \({\frac{{3 \times {{10}^8}}}{{1.5}} = 2\,{\text{m}}/{\text{s}}}\)

As \(\mu=\frac{1}{\sin C}\)

\(\therefore \sin C=\frac{1}{\mu}=\frac{1}{2}=30^o\)

Condition of \(TIR\) is angle of incidence i must be greater than critical angle. Hence ray will suffer \(TIR\) in case of \((B)\) \(\left(i=40^o > 30^o \right)\) only.