10 % rule or law of energy transfer that only 10% of the energy stored in the organisms of each trophic level is available as a food for the next trophic level and large amount of energy is used by the organism itself and lost as heat. In this question, if biomass is present in one jack pine (producer) is 12000 kg/m²/year then biomass at primary consumer will be 10% of 12000 kg would be 1200 kg similarly at secondary level will be 120 kg and finally at tertiary level biomass would be 12 kg/m²/year.