The correct answer is option 3: Thermal power plants.

Thermal power plants are the largest source of electricity in India. These plants generate electricity by using coal, oil, or natural gas to produce steam that drives turbines and generates electricity.

Option 1: Hydropower plants are a source of electricity in India, but they are not the largest source. Hydropower plants generate electricity by harnessing the energy of flowing or falling water and converting it into electricity. Although India has several hydropower plants, they do not contribute as much electricity as thermal power plants.

Option 2: Nuclear power plants are another source of electricity, but they are not the largest source in India. Nuclear power plants generate electricity by harnessing the energy released from nuclear reactions. While India does have nuclear power plants, their contribution to electricity generation is not as significant as that of thermal power plants.

Option 4: Wind energy is also a source of electricity, but it is not the largest source in India. Wind energy is generated by harnessing the power of wind turbines. India has been increasingly investing in wind energy, but it does not produce as much electricity as thermal power plants.

In conclusion, thermal power plants are the largest source of electricity in India, making option 3