The difference is that "Net production is always less than gross production".
Option (b);
EXPLANATION:
Net production is the rate at which all plants, algae, bacteria produce net chemical energy in the useful ecosystem. The formula statement is that Net Production = Gross Production - Respiration by plants.
Whereas, Gross production is the total sum of energy that is produced by the producer. It is the energy in that area in a given amount of time. Gross production doesn't include the energy that is lost in respiration. Thus the production value is less than gross production.