Answer:
b. One electron state is an anti-bonding orbital, which results in an absence of electron density between atoms.
Explanation:
Due to heat , the kinetic energy of the atoms in the substance increases due to which they start vibrating vigorously and the inter-molecular attraction decreases and the substance increases in volume. For example, on boiling water...heat energy imparted makes the volume increase and finally turn into steam or water vapour in air.(The inter-molecular space in air is more than that in water.)