Which of the following is primarily responsible for limiting the number of trophic levels in most ecosystems?
(a) decomposition limits nutrient cycling rates.
(b) energy transfer between trophic levels is almost always less than 20% efficient.
(c) nutrient cycles involve both abiotic and biotic components of ecosystems.
(d) direct competition between consumers and decomposers for nutrients and energy.
(e) resource partitioning among higher-order consumers.

(B) Energy transfer between trophic levels is almost always less than 20% efficient.

Explanation:

The ultimate source of energy on the Earth is the Sun. The energy coming from the Sun is captured by green plants by the photosynthesis. During photosynthesis sun energy   is fixed into chemical energy (carbohydrate). So, in an ecosystem energy flow is unidirectional (from sun to the green plants). The fixed chemical energy from green plants is transferred to the herbivores then to carnivores through food. When one organism eats another organisms, only 10 % of the energy present in the organism is transferred as a food for the next organism and a large amount of energy is lost as heat into the environment. Thus, energy keeps on decreasing when stored energy moves from producers to top consumers. Thus, less than 20% energy transfer limits the trophic levels in most of the ecosystem.    

your answer should be c) enjoy!