Which of these of these organisms would most likely be found at the bottom of biomass pyramid?

It seems as though you did not provide answer choices, but a biomass pyramid starts at the primary producers, which then moves up to primary consumers, secondary consumers, and all the way to detritivores. Primary producers are organisms like plants that convert light energy (or other energy) into biomass that other organisms can use.

A. mitochondrion b. nucleus

The correct answer is  b)