According to API RP 571:
"Ammonium chloride corrosion is most aggressive when dry salts are exposed to a small amount of free water. This condition allows the formation of a concentrated acidic aqueous solution, which is highly corrosive to carbon steel."
"The corrosion mechanism is activated especially during startup and shutdown when condensation may occur on salt deposits." (Reference: API RP 571, Section 4.3.3.1 - Ammonium Chloride Corrosion) Hence, while salt deposition begins as temperatures drop, the most aggressive corrosion happens when water is introduced to dry salt, making option D correct.