Correct option is (2) 10 cm
For an observer outside the water,
Height of water = \(\frac x{\mu_W}\)
\(= \frac x{\frac 43 }\)
\(= \frac{3x}4\)
Height of air observed by observer = 17.5 – x
According to question,
\(\frac{3x}4 = 17.5 - x\)
⇒ \(\frac{7x}4 = 17.5\)
⇒ \(x =10\) cm