Which substance has the highest melting point? a. h2o b. hcl c. ch4 d. mgcl2

A.H2O = water, so it is a liquid at room temperatures (melting point = 0C) and melting point must be below 20C

B.HCl = A gas at room temperatures, so melting point (-114C) is definitely lower than 20C.
Melting points of the aqueous solution (solution in water) at different concentrations are all below that of water, i.e. 0C.

C.CH4 = A gas at room temperatures, so melting point (-182C) must be below 20C.

D.MgCl2 = A white powdery solid at room temperatures, so its melting point (714C) must be above 20C (So this is the only one among the four choices that melts above 20 ° C).