Master plumbers must be ____________ all work performed which is permitted under his license.

The correct choice highlights that master plumbers must be knowledgeable of and responsible for all work performed that is permitted under their license. This responsibility is integral to the role of a master plumber, as it encompasses not only the technical skills required to perform plumbing tasks but also the necessary understanding of plumbing codes and standards.

Master plumbers are expected to have a comprehensive understanding of the plumbing systems they work with and the regulations governing those systems. This knowledge ensures that all installations, repairs, and maintenance are conducted safely, efficiently, and in compliance with local codes. Moreover, being responsible means that they cannot delegate their accountability; they must ensure that the work is executed properly, and any decisions made during the project fall under their purview.

While the other options might suggest various requirements that could be related to plumbing work, such as needing external certification or client approval, they do not capture the essence of the master plumber's role, which fundamentally revolves around their knowledge and responsibility for the work performed. This ensures that they maintain the standards of the profession and protect the health and safety of the public through competent plumbing practices.