c
Apparent depth \( = \frac{x}{{{\mu _1}}} + \frac{x}{{{\mu _2}}} = x\)

\( \Rightarrow \,\,\,\,\,\frac{1}{{{\mu _1}}} + \frac{1}{{{\mu _2}}} = 1\)

\( \Rightarrow \,\,\,\frac{1}{3} + \frac{1}{\mu } = 1\) \( \Rightarrow \,\,\,\mu = 1.5\)