Final answer:
Argon (Ar) has the smallest dispersion forces among the substances listed because it is a single noble gas atom with a low molar mass, leading to weaker intermolecular attractions. Option A is correct.
Step-by-step explanation:
Out of the substances Ar, Cl₂, CCl₄, and HNO₃, the one with the smallest dispersion forces is A) Ar, argon. Dispersion forces, also known as London dispersion forces or Van der Waals forces, depend on the size and molar mass of the molecule or atom.
Since Ar is a noble gas with a single atom and a relatively low molar mass compared to the molecular substances listed, it has the weakest dispersion forces. This is because Ar is smaller and lighter, resulting in less polarizability and therefore weaker dispersion forces. In contrast, Cl₂, CCl₄, and HNO₃ are larger molecules with stronger dispersion forces due to their greater electron cloud distortion capabilities.