Final answer:
Acetic acid is less likely than hydrochloric acid, sulfuric acid, or nitric acid to completely dissolve in water, as it is a weak acid and does not fully dissociate into ions.
Step-by-step explanation:
The substance least likely to completely dissolve in water from the given options is acetic acid (CH3CO2H). Hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3) are all strong acids that completely dissociate into their constituent ions in water. In contrast, acetic acid is a weak acid, meaning that when it dissolves in water, it does not fully dissociate into ions; a significant proportion of the acetic acid molecules remain undissociated in solution. Hence, acetic acid is less likely than the other options to be completely dissolved in water in the form of ions.