In general, which trophic level has the least energy available to it?

The correct answer is - the tertiary consumers.

Explanation:

The energy amount decreases at each tropic level as it moves in the ecosystem. At every trophic level 10% of the energy transferred to the next level, rest energy utilize in the metabolic process and lost as heat.

In a normal ecosystem normally, maximum energy is present at the producer level, from producers the ten percent of the energy transfer to the primary consumer and from them ten percent moves to the secondary consumers. The ten percent of the energy present moves ultimately to the tertiary consumer.

Thus, the correct answer is - tertiary consumers.