ANSWER
a. solids
Step-by-step explanation
Intermolecular forces are the forces that keep the bond between the molecules of a substance. The strongest these forces are, the hardest is to change the shape of the substance.
So, since gases don't have a fixed form because they adapt to where they are contained, these are the ones with the least intermolecular bonding.
Liquids have a stronger intermolecular bonding than gases because their volumes don't change, but their shapes do.
Finally, solid substances have a fixed shape, so their intermolecular bonding is higher than gases and solids.
Hence, solids have the greatest intermolecular bonding.