Which of the following statements about biofuels is true?
a. biofuels are composed of 100% renewable materials.
b. algae is the leading source of biofuels.
c. energy processing from corn biofuels is not energy efficient.
d. corn is the only source of biofuels.

The correct answer is C. 
Energy processing which is from corn biofuels is not energy efficient.

Biofuels is termed as a fuel which is being produced through contemporary biological processes, for example, anaerobic digestion and agriculture. A fuel which is produced by geological processes includes the one which is involved in the formation of fossil fuels. 
For example, petroleum and coal, which are from prehistoric biological matter. Biofuels can be found directly from plants. There are renewable biofuels which involve contemporary carbon fixation like those occurs in the plants.