Which if the following is a correct description of cogeneration?

a. the process of recovering waste thermal energy produced in electricity generation.

b. the process of using crop waste to produce ethanol.

c. the process of recycling cooking oil and reprocessing it into biodiesel.

d. the process of using the steam generated by a nuclear power plant to create electricity.

Cogeneration is the process of taking a waste product or process and producing heat and generating electricity from it at the same time. 
It could be collection of waste heat from the original generation of electricity or burning of biodiesel, ethanol or methane gas from sewer or cow dung. The only answer where electricity is using or creating heat and generating electricity power is A.