Which is the least sustainable practice.
a. finding new sources of geothermal energy
b. building more windmills
c. using coal to generate electricity
d. installing more solar panels

C. Using coal to generate electricity

Explanation:

Sustainable practice can be define as the practice which ensures the use of resources in a manner so that these resources remain available for the present as well as for the future generations.

C. is the correct option this is because of the fact that coal is the non-renewable resource. The excess use of coal for generation of electricity is likely to limit the concentration of coal available in a region. Hence, the use of coal cannot be considered as sustainable practice.      

explanation:

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.

the real answer is b. scintillation counter