Which of the following has melting point below $500°\mathrm{C}$?

The melting point of cadmium (Cd)  is 321.04℃. The melting point of  Copper (Cu)  is 1084℃. The melting point of  Zinc (Zn)is 419℃. The melting point of silver (Ag)  is 961℃. Hence, the elements with a melting point below 500 ℃ are Cd and Zn.

Because their orbitals contain a vacant d-orbital, transition metals have relatively high melting points in the metallic bonds they create. They form a strong metallic bond that takes a lot of energy to break and have a high melting point because they have a vacant d-orbital and a very high heat of atomization.
The last element in each period, however, has a very low melting point because its d-orbital is completely filled, and as a result, it cannot form metallic connections with other elements. They, therefore, have a low melting point and low heat of atomization. Cadmium and zinc are the final elements of the 3d and fourth4d series, respectively.

Hence, the correct answer is option B.