Answer
c. 640 kilometers per hour.
Work Step by Step
As we know that
velocity of tsunami for depth d is $v=\sqrt{gd}=450\frac{Km}{h}$
so velocity of tsunami for depth 2d is $v^{\prime}=\sqrt{2gd}$
$\implies v^{\prime}=\sqrt{2} \space gd$
$\implies v^{\prime}=\sqrt{2}\times 450\approx 640\frac{Km}{h}$
