Correct option is (c) \( \frac{30} {\sqrt 2}\) kmh-1
\(\vec{v_{rg}} = \vec {v_r} - \vec {v_g}\)
From the above diagram, velocity of the raindrop wrt ground will be,
\(|\vec{v_r}| = \sqrt{(15\sqrt 2)^2 + (15\sqrt 2)^2}\)
\(= 30 \) kmh-1
And velocity of the raindrop wrt the moving girl will be,
\(|\vec {v_{rg}}| = 15\sqrt 2\)
\(= \frac{30} {\sqrt 2}\) kmh-1