Which of the following weldments is most susceptible to dissimilar metal cracking when operating at...

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.