What pertains to the alteration, repair, or extension of plumbing systems in a building?

The focus of the inquiry is specifically on the alteration, repair, or extension of plumbing systems within a building. The correct answer directly addresses this scope by categorizing plumbing activities that involve modifications to existing systems, such as expanding plumbing infrastructure, making adjustments to materials used, or executing repairs on current plumbing lines. This definition encompasses a wide range of activities that fall under the umbrella of plumbing maintenance and enhancement in buildings.

Other options presented do not encompass the full range of activities outlined in the question. New plumbing installations refer to the process of setting up entirely new plumbing systems rather than modifying existing ones. Sewage system installation focuses specifically on the construction or integration of sewage handling systems, which is a subset of plumbing work but does not cover repairs or alterations to existing plumbing. Waterproofing enhancements are relevant to the protective measures against water ingress but do not pertain directly to plumbing systems and their alterations. Therefore, the selected answer is the most comprehensive in representing the scope of work regarding plumbing modifications.