The correct answer is: a) Micro and mini hydel projects.
Sikkim has a policy to promote small-scale hydropower projects, which are known as micro and mini hydel projects. These projects are designed to generate electricity from the flow of water in rivers and streams. They are typically smaller than large hydropower projects, and they can be located in remote areas where there is no access to the electricity grid.
Micro and mini hydel projects have a number of advantages over large hydropower projects. They are less expensive to build and operate, and they have a smaller environmental impact. They can also be used to generate electricity in areas where there is no other source of power, such as remote villages and mountain communities.
Sikkim has a number of micro and mini hydel projects in operation, and the government is encouraging the development of more projects. The government has also set a target of generating 100 megawatts of electricity from micro and mini hydel projects by 2020.
The other options are incorrect because they are not types of small-scale hydropower projects. Solar rooftop installations are a type of renewable energy project, but they are not hydropower projects. Wind energy farms and biogas plants are also renewable energy projects, but they are not small-scale hydropower projects.