Final answer:
A person welding gas piping is required to have certification by a recognized agency, which ensures they are trained in safety procedures and proper welding techniques for combustible or hazardous gases.
Step-by-step explanation:
The question pertains to the qualifications required for a person welding gas piping. The correct answer is c) Certification by a recognized agency. Welding gas piping is a task that involves safety risks; therefore, it requires specific training and certification. Certifying agencies ensure that welders understand the properties of the gases they are dealing with, as well as the safety procedures and techniques for welding pipes that will be carrying combustible or potentially hazardous gases. A welding certificate alone (option a) or completion of an apprenticeship (option b) might be part of the process to become certified, but they do not replace the need for specialized certification. Furthermore, it is not permissible to weld gas pipelines without any specific qualifications (option d).