Which factor is generally responsible for high melting points?

The factor that is generally responsible for higher melting point is intermolecular forces. The compounds that are covalent in nature are made of molecules rather than ions. It has been seen that some of the covalent compounds have polar molecules at one end, due to which the one end has more electronegative force than the other. The electrostatic force that is bounding the compound is the main cause of higher melting point of this compound.  So it is true that with the increase of polarity of a compound creates higher melting point. .. hope I helped

And a) oxygen i believe

Idon't know hope someone you