The imperfect differential is a thermodynamic parameter that is not exact in its differential form, which means that the path taken during the process will affect its value. Only state functions are exact differentials, meaning their values depend only on the initial and final states, not on the path taken between them.
Among the given options, dQ (change in heat) is an imperfect differential because it depends on the path taken by the system. The heat absorbed or released by the system depends on the temperature difference between the system and its surroundings, and this temperature difference can vary depending on the path taken by the system during the process. In contrast, dH (change in enthalpy), dS (change in entropy), and dG (change in free energy) are all state functions and exact differentials because their values depend only on the initial and final states of the system, not on the path taken between them.