What is an explanation of why producers are always found at the lowest trophic level?

This answer is probably late, but the answer that you put is right :) ( C.)

Explanation:

The reason why producers are always found at the lowest trophic level is because there is no need for them to consume other organisms (or living things) to be able to get their energy.

In other words, they do not need to consume living things to produce energy. They make their own food.

Hope this helps! :)