Why does hexaphenylbenzene have a high melting point?

The melting point is a feature of inter-molecular forces of attraction between molecules of a substance

The bigger the size of a molecule more the van der waal forces between molecules and more the attraction and hence more the melting point and boiling point of any substance.

If we look at the structure of  hexaphenylbenzene we will be able to realize that it is a big molecule with a benzene ring having six more benzene ring (as shown in the diagram)

Thus it has high melting point

the transfer of heat by electromagnetic radiation is called thermal radiation . it is usually carried out by electromagnetic waves in an energy range just below the visible spectrum, which is the infrared range.

so the answer would be radiation.

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