Which of the following processes is NOT suitable for the construction of boilers?

D is the correct choice because soldering is not suitable for the construction of boilers. In the context of boiler fabrication, the joints must withstand high pressure and temperature conditions. Soldering involves the use of a filler metal that has a melting point below 450°C (842°F), which does not provide sufficient strength or durability in the high-stress environments typically found in boilers.

On the other hand, processes such as arc welding, brazing, and submerged arc welding all involve the use of filler materials or techniques that can create strong, high-integrity joints capable of withstanding the operational demands of a boiler. These methods produce permanent and robust connections that can endure the stresses of pressure and heat that boilers are subjected to in service.