Let the vertical downward direction be along $-\hat{j}$ and the horizontal direction of the girl's movement be along $\hat{i}$. Thus, the velocity of the rain is $\vec{v}_r = -4\hat{j}$ and the velocity of the girl is $\vec{v}_g = 3\hat{i}$. The velocity of the rain relative to the girl is $\vec{v}_{rg} = \vec{v}_r - \vec{v}_g = -3\hat{i} - 4\hat{j}$. The magnitude of this relative velocity is $|\vec{v}_{rg}| = \sqrt{(-3)^2 + (-4)^2} = \sqrt{9 + 16} = 5\text{ km/h}$. Option 2 ($7\text{ km/h}$) and option 3 ($1\text{ km/h}$) are algebraic additions/subtractions which do not apply to perpendicular vector quantities.