What law accounts for the decrease of energy at each trophic level?

Second law of thermodynamics

Explanation:

The second law of thermodynamics states that whenever energy is transformed, there is a loss energy through the release of heat. This occurs when energy is transferred between trophic levels as illustrated in a food web. When one animal feeds off another, there is a loss of heat (energy) in the process.

the correct answer is:

a population changes in response to a change in the environment.