Correct Answer - C
As, we know number of waves in glass slab
`=` number of waves in water column
where, `mu_(g) =` refractive index in glass,
`mu_(w) =` refractive index of water column,
`h_(g) =` thickness of slab and `h_(w) =` height of water column.
Given, `mu_(g) = 15, h_(g) = 6 cm, h_(w) = 7` cm
`rArr mu_(w) = (mu_(g).h_(g))/(h_(w)) = (15 xx 6)/(7)`
`:. mu_(w) = 9/7 = 1.286`