Assertion (A) : Pyramid of energy is always upright, can never be inverted.
Reason (R) : When energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step.

Both Assertion (A) and Reason (R) are correct, and Reason (R) is the correct explanation for Assertion (A).

The pyramid of energy is always upright, meaning that energy decreases as you move up trophic levels in an ecosystem. This is because when energy is transferred from one trophic level to the next (e.g., from producers to herbivores, then to carnivores), some energy is always lost as heat due to metabolic processes. This loss of energy at each step ensures that the pyramid of energy remains upright, with each higher trophic level containing less energy than the one below it. Thus, Reason (R) provides a valid explanation for Assertion (A).