In API RP 571, Carbonate SCC and Alkaline SCC are both discussed under mechanisms that occur in basic (high pH) environments, often with similar contributing conditions and prevention methods. The document highlights:
"Carbonate SCC is considered a subcategory of alkaline stress corrosion cracking. It results from the concentration of alkaline carbonates under thermal insulation or deposits."
"Both mechanisms involve the same key parameters: elevated temperatures, tensile stress, and high pH solutions." (Reference: API RP 571, 3rd Edition, Section 4.2.2.4 and 4.2.2.3) Because these mechanisms share similar material susceptibility and operating conditions, they are indeed closely related-option B is therefore the accurate and supported answer.

According to API RP 571 and API RP 751 (HF Alkylation Units):
"HF acid corrosion rates increase with elevated temperature and higher water content. This is because water acts as an ionizing agent that facilitates the acid's attack on steel."
"Corrosion is minimized when water content is strictly controlled, usually between 0.5-1.5%."
"Contrary to some assumptions, increasing HF acid concentration actually decreases corrosivity." Hence, option B is correct.

API RP 571 under Sulfidation (High Temperature Sulfidic Corrosion) recommends:
"Thickness monitoring using ultrasonic testing (UT) or radiographic testing (RT) is the most common method for detection of wall loss due to sulfidation."
"Sulfidation results in uniform thinning, especially in low Cr steels in hydrogen/H#S environments." (Reference: API RP 571, Section 4.2.1.1 - Sulfidation) Thus, option A is the most appropriate and effective inspection method.

Dissimilar Metal Cracking (DMC) is a form of high-temperature cracking that frequently occurs in weld joints between carbon steel and austenitic stainless steel (such as Type 316 SS) due to differences in thermal expansion coefficients and mechanical properties.
From API RP 571 Section 5.2.3.1 (Dissimilar Metal Weld Cracking):
"Cracking frequently occurs in weld joints between ferritic and austenitic materials such as carbon steel to
300 series stainless steels due to thermal expansion mismatch during high temperature operation." Therefore, Option D (Carbon steel to 316 stainless steel) is the most susceptible combination and the correct answer.

From API RP 571, brittle fracture prevention measures include:
"Postweld Heat Treatment (PWHT) reduces residual stresses in the heat affected zone and base metal, thus lowering susceptibility to brittle fracture."
"PWHT is particularly effective when applied to pressure-containing components fabricated from carbon steel or low-alloy steel that will experience low-temperature service." (Reference: API RP 571, Section 4.2.1.2 - Brittle Fracture) Therefore, while other measures may reduce stress or detect flaws, PWHT directly targets one of the root causes: residual stress. Thus, option D is the best prevention method.