API RP 571 on Organic Acid Corrosion in Overheads explains:
"Corrosion caused by organic acids (e.g., formic, acetic) in overhead systems is best mitigated by neutralization, usually via injection of alkaline neutralizing amines that raise pH and reduce acidity."
"Water wash can be helpful but is often insufficient by itself."
Therefore, the most effective approach is option C - injection of neutralizer.

According to API RP 571 Section 5.3.2.1 (Creep):
"Creep damage is exponentially dependent on temperature. A small increase in temperature (e.g., 25°F or 15° C) can reduce remaining life by more than half. This is because the creep rate increases rapidly with temperature, especially above design limits." Stress is a contributing factor, but the dominant and most sensitive variable is temperature.
Thus, Option C (increase in temperature of 25°F/15°C) is the correct answer.

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.

According to API RP 571, in the section on "Chloride Stress Corrosion Cracking (Cl-SCC)", several critical factors are outlined that contribute to the initiation and propagation of this mechanism. The document states:
"Critical factors include the presence of chlorides (even at ppm levels), oxygen, elevated temperatures, tensile stress, and susceptible materials such as austenitic stainless steels and some nickel alloys."
"Oxygen is an accelerant and promotes pitting that can lead to SCC initiation." (Reference: API RP 571, 3rd Edition, Section 4.2.2.2) Therefore, the presence of oxygen is a well-documented critical factor in Cl-SCC. It acts in conjunction with chlorides to initiate localized pitting, which then progresses into cracking. Hence, option B is the correct choice.

According to API RP 571 and API RP 751:
"In HF alkylation units, carbon steel can be used if weld hardness is controlled to # 200 Brinell (BHN).
Exceeding this can significantly increase the corrosion rate and susceptibility to cracking."
"Hard welds act as preferential corrosion sites due to microstructural inhomogeneity and stress concentration." (Reference: API RP 571, Section 4.3.3.4 - Hydrofluoric Acid Corrosion; API RP 751, Section 5.3 - Materials of Construction) Thus, option D is correct.