In an ecosystem, a deer consumes grass. what percentage of energy will the deer acquire from the grass?

10% of total energy available in grass.

Explanation:

The energy transfer in an ecosystem occurs through the food chain and follows the 10% law and the second law of thermodynamics. Accordingly, during the transfer of energy from one trophic level to other, only 10% of energy is transferred and the rest 90% is lost as metabolic heat.  

Hence, the grass will use 90% of its total energy content in respiration and this energy will be lost as heat to the surroundings. Only 10% of the energy of grass will be available for deer.