What physical quantity is the same for x- rays of wave length $10^{-10}m$ red light of wave length $6800A^{\circ}$ and radio waves of wave length $500\;m$?

Question

$\begin{array}{1 1} 1.5 \times 10^8\;ms^{-1} \\ 3 \times 10^8\;ms^{-1} \\ 6 \times 10^8\;ms^{-1} \\ none\;of\;the\;above \end{array}$