Among the following, which trade is a locksmith?

The trade of locksmith focuses on the skill of working with locks and security systems. Locksmiths are trained professionals who install, repair, and maintain locks and security devices. They possess knowledge of various locking mechanisms and often provide services such as key duplication, lockout assistance, and the installation of advanced security systems.

This profession requires specialized tools and an understanding of both mechanical and electronic lock systems, which distinguishes it clearly from the other listed trades. Painters and decorators, appliance service technicians, and welders pursue entirely different paths involving painting, repairing household appliances, and fabricating or repairing metal structures, respectively. Each trade has its unique focus and skill set, further reinforcing that the locksmith is the only choice that directly matches the description provided in the question.