The likelihood of brittle fracture is best lessened by:

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.