In comparison to molecules that interact by london dispersion forces only, the melting point of similar-sized molecules forming hydrogen bonds would most likely be
about the same.
unpredictable.
lower.
higher.

Molecules forming hydrogen bonds will have higher melting point.

Explanation:

Decreasing order of intermolecular forces in terms of their strength:

Ion-on > Hydrogen bond > dipole-dipole > London dispersion force

Hydrogen bonding is a combination of pure ionic bond and pure covalent bond. London dispersion force arises from weak dipole dipole interaction created due to polarizability of electrons during rotation around nucleus.

Hence molecules forming hydrogen bond will have higher intermolecular force. Therefore more temperature is required to melt it by disrupting intermolecular hydrogen bonds.

So, molecules forming hydrogen bonds will have higher melting point.

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.