Explanation

(d) $\mu = \frac{\sin\left( \frac{A + \delta_{m}}{2} \right)}{\sin\frac{A}{2}}$
For equilateral prism, $\angle A = 60{^\circ}$
$\mu = \frac{\sin\left( \frac{60{^\circ} + 60{^\circ}}{2} \right)}{\sin\frac{60{^\circ}}{2}} = \frac{\sin{60{^\circ}}}{\sin{30{^\circ}}} = \frac{\frac{\sqrt{3}}{2}}{\frac{1}{2}} = \sqrt{3} = 1.73$