b
(b)Angular fringe width \(\theta = \frac{\lambda }{d} \Rightarrow \theta \propto \lambda \)
\({\lambda _w} = \frac{{{\lambda _a}}}{{{\mu _w}}}\)
So \({\theta _w} = \frac{{{\theta _{{\rm{air}}}}}}{{{\mu _{\rm{w}}}}}\) \( = \frac{{0.20}}{{\frac{4}{3}}} = 0.15^\circ \)