API RP 571 provides the following guidance regarding temper embrittlement:
"For materials that contain impurity levels which make them susceptible to temper embrittlement, the most effective preventive measure is to specify low FATT or Charpy impact energy requirements."
"These requirements ensure the material retains adequate toughness even if embrittlement occurs."
"While temper embrittlement itself cannot always be fully prevented during long-term exposure to temperatures between 650°F and 1070°F (343°C-577°C), its effects can be identified and mitigated through impact testing criteria." Therefore, option C is correct. Specification of Charpy V-notch testing ensures material selection accounts for embrittlement susceptibility and maintains service integrity.

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.

According to API RP 571:
"Oxidation is the reaction of metal and oxygen to form oxides. It generally occurs in high temperature environments with excess oxygen."
"Sulfidation is a high temperature corrosion mechanism that occurs due to interaction between sulfur- containing compounds and metal surfaces, forming metal sulfides. Like oxidation, sulfidation occurs in environments above 500 °F (260 °C)."
"Both oxidation and sulfidation can occur concurrently, particularly in mixed environments of oxygen and sulfur." (Reference: API RP 571, Sections 5.1.1 and 5.1.3 - Oxidation & Sulfidation) These two mechanisms are thermally activated and occur in elevated temperature conditions found in heaters, furnaces, and piping systems. Hence, option B is correct.

API RP 571 - Sulfidation:
"Sulfidation generally results in uniform thinning of iron-based alloys at temperatures above 500°F (260° C)."
"It is classified as a form of general corrosion, although it can be accelerated in turbulent flow areas." So, option A is correct.

From API RP 571 Section 5.3.3.2 (Caustic Corrosion):
"Caustic corrosion, also known as caustic gouging, occurs in boiler systems when caustic concentrates in under-deposit areas or due to improper steam drum and boiler water design. Good design and water treatment practices are key preventive measures." Temperature control or acid injection are not practical or effective mitigation techniques. Design considerations-such as proper water flow, material selection, and minimizing deposits-are essential.
Hence, the correct answer is Option C.