The correct answer is option 'C' - Construction of a dam.

The tertiary sector, also known as the service sector, is an important sector of the economy that focuses on providing services rather than producing goods. It includes a wide range of activities such as transportation, healthcare, education, tourism, banking, retail, and construction.

Construction of a dam falls under the category of the tertiary sector because it involves the provision of services rather than the production of goods. Let's explore this in more detail:

Construction of a Dam:
1. Infrastructure Development: Building a dam involves the construction of a large-scale infrastructure project. It requires a range of services such as architectural design, engineering, project management, and skilled labor.
2. Project Planning and Management: The construction of a dam requires careful planning and management to ensure the project is completed efficiently and within budget. This involves activities such as feasibility studies, environmental assessments, cost estimates, and procurement of materials and equipment.
3. Skilled Labor: Construction of a dam requires a skilled workforce including engineers, architects, construction workers, electricians, and plumbers. These professionals provide specialized services to ensure the successful completion of the project.
4. Ancillary Services: In addition to the direct construction activities, there are various ancillary services involved in the construction of a dam. These include transportation of materials and equipment, provision of accommodation for workers, security services, and catering services.
5. Operation and Maintenance: Once the dam is constructed, it requires ongoing operation and maintenance services. This includes managing the water flow, monitoring the structural integrity of the dam, and conducting regular inspections and repairs.

In summary, the construction of a dam involves a range of services provided by professionals and skilled labor. It is an activity of the tertiary sector as it focuses on the provision of services rather than the production of goods.