Which one is the possible prime source of energy in Nepal?

The correct answer is D : hydro power 

Hydropower is using water to power machinery or make electricity. ... Turbines and generators convert the energy into electricity, which is then fed into the electrical grid to be used in homes, businesses, and by industry.Because hydropower plants can generate power to the grid immediately, they provide essential back-up power during major electricity outages or disruptions. In addition to a sustainable fuel source, hydropower efforts produce a number of benefits, such as flood control, irrigation, and water supply.

Hydropower is the most important and widely-used renewable source of energy. Hydropower represents about 17% (International Energy Agency) of total electricity production. China is the largest producer of hydroelectricity, followed by Canada, Brazil, and the United States.Hydropower is the only renewable source of energy that can replace fossil fuels' electricity production while satisfying growing energy needs.” ... Although hydropower is an inexpensive and environmentally friendly form of energy, it is often overlooked as an energy source.