The most effective means of preventing caustic stress corrosion cracking is:

According to API RP 571, under Caustic Stress Corrosion Cracking (Caustic Embrittlement):

“Cracking occurs in stressed components exposed to caustic (e.g., sodium hydroxide).”

“The most effective mitigation strategy is postweld heat treatment (PWHT), which relieves residual stress that promotes cracking.”

“Upgrading materials or controlling concentration may help, but do not replace stress relief.”

(Reference: API RP 571, Section 4.2.2.3 – Caustic SCC)

Thus, PWHT is the most effective prevention technique, making option A correct.