Which of the following solids would possess the lowest melting point?

c25h52

hg

cacl2

cuo

 Hg

Explanation:  As we know that in case of hydrocarbons, as the size increases, vander waal forces become strong thus attract each other with a strong tendency thus C_{25}H_{52} is solid at room temperature. Its melting point is approximately 50°C.

Mercury is the only metal which exist as liquid at room temperature thus iy has lower melting point of  -38.83 °C.

CaCl2 is and CuO are closely packed solids and thus have high melting point. The former one has melting point of 772 °C and th latter one has melting point of 1326°C.

Thus Mercury has lowest melting point out of the given options.