Which of the following is a conventional source of energy?

Currently, fossil fuels are a good source when compared to wind, tidal and solar energy, as they are less expensive, easy to store and access and efficient. However, they are not environmentally friendly. Day by day they are being consumed at an alarming rate and would soon run out and hence would not remain a good source of energy. Whereas, due to the invention of technologies, wind, solar and tidal energies are becoming more economical and easier to store and transfer and would soon replace fossil fuels.