Consider the actual height of the tumbler be H.

The refractive index of the water, $$\mu$$ = 4/3

The refractive index of the air, $$\mu$$ = 1

As we know that,

$${\mu _{water}} = {{{H_{real}}} \over {{H_{apparent}}}} \Rightarrow {4 \over 3} = {H \over {{H_{apparent}}}}$$

$$ \Rightarrow {H_{apparent}} = {{3H} \over 4}$$

Height of air observed by observer = 17.5 $$-$$ H

According to question, both height observed by observer is same.

$${{3H} \over 4}$$ = 17.5 $$-$$ H

$$\Rightarrow$$ H = 10 cm

Option (b)