Assertion: In lower trophic levels a greater number of individuals are present.

Reason: Energy moves in a single direction.

In an ecosystem, there are generally more people at the lower trophic levels; the producers make up the majority. An ecosystem's energy flow is always linear or unidirectional. It is not possible to return the energy from producers to the solar input. Additionally, the energy that is transferred to herbivores is not returned to autotrophs.

Hence, the correct option is b) Both A and R are true, but R is not the correct explanation of the assertion.