Energy that is not passed on from one trophic level to the next is

a.
lost as heat
b.
returned to the sun
c.
destroyed
d.
all of the above

The correct answer for the blank is a.  lost as heat.

Trophic level corresponds to a particular rank of an organism in a food chain within an ecosystem ( such as producers and primary consumers occupy the first trophic and the second trophic level respectively).

According to the 10 percent rule of energy transfer, only 10% of the energy ( in the form of organic matter) is transferred from one (lower) trophic level to the next (higher) trophic level and the remaining is utilized in the cellular processes and lost as heat in the surrounding.

Thus, option A) is the right answer.