Which of the following is the least expensive? 

A)  hydroelectricity 
B)  tidal energy 
C)  improving energy efficiency 
D)  photovoltaics

Hydroelectricity

Electricity generated by harnessing the energy of falling or flowing water, typically by using dams or diversion structures to drive water turbines and generators.

Tidal Energy

Renewable energy obtained by harnessing the movement of tides, typically through the use of tidal turbines or barrages, converting kinetic energy into electricity.

Energy Efficiency

The strategy of using less energy to provide the same service or perform the same task, which helps in reducing energy waste and improving economic efficiency.