A driver looking up through the water sees the outside world contained in a circular horizon .The refractive index of water is $\;\large\frac{4}{3}\;$ , and the diver's eyes are 15 cm below the surface water . Then the radius of the circle is :

Question

$(a)\;15 \times3 \times \sqrt{5}\;cm\qquad(b)\;15 \times3 \times \sqrt{7}\;cm\qquad(c)\;\large\frac{15 \times \sqrt{7}}{3}\;cm\qquad(d)\;\large\frac{15 \times 3}{\sqrt{7}}\;cm$