The best explanation for why there are fewer secondary consumers than producers is:
Around 10% of the energy is transferred to support the next trophic level.
This statement reflects the concept of energy transfer in ecological pyramids, where only a small fraction (approximately 10%) of the energy from one trophic level is available to the next. As a result, fewer organisms can be supported at higher trophic levels, leading to a smaller number of secondary consumers compared to producers.