Final answer:
Among the substances listed, glass is the least fluid due to its rigid amorphous solid structure and very strong intermolecular forces, making it essentially non-flowing under normal conditions.
Step-by-step explanation:
Intermolecular forces are forces of attraction or repulsion which act between neighboring particles (atoms, molecules or ions). They are one of the primary factors determining the physical properties of a substance, including its fluidity. When comparing the substances mercury, mercury glass, glass, and rubber in the context of which one would be the least fluid, we must consider the kind and strength of the intermolecular forces they exhibit.
Glass, which is an amorphous solid, exhibits very strong intermolecular forces in the form of covalent or ionic bonds that keep its structure rigid and firmly held in place, making it the least fluid among the options. While mercury, a liquid at room temperature, displays metallic bonding, and mercury glass (a mixture of mercury with glass) and rubber have weaker intermolecular forces compared to solid glass. Rubber is elastic and can be deformed but still has stronger intermolecular forces than liquids, allowing it to have some shape retention.