During inspection of a process unit where sulfidation from high-temperature hydrogen exists in H₂S streams,...

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/H₂S environments.”

(Reference: API RP 571, Section 4.2.1.1 – Sulfidation)

Thus, option A is the most appropriate and effective inspection method.