We know that when we view from underwater.

$\mu =\large\frac{apparent \; height }{real \;height}$

Apparent height from surface $= 36-12=24 m$

$\mu =\large\frac{4}{3}$

$\large\frac{4}{3}=\frac{24}{real\;height}$

Real height $= 18\;m$

Hence c is the correct answer.