Which of the following ecological pyramids is always upright?
Correct: C
The pyramid of energy is always upright because energy is lost at each successive trophic level through metabolic processes and heat loss, following the first law of thermodynamics. Unlike pyramids of numbers or biomass, which can be inverted or spindle-shaped depending on the ecosystem (e.g., a single large tree supporting many insects gives an inverted pyramid of numbers; a forest ecosystem can show an inverted pyramid of biomass), the pyramid of energy always decreases from producers to top consumers.