The correct answer is option 3: Decrease in the available energy at higher trophic levels.

Trophic levels refer to the feeding positions in a food chain. A food chain consists of producers (plants), consumers (animals), and decomposers (bacteria and fungi). Energy flows through these trophic levels as one organism consumes another.

However, the available energy decreases as it moves up the food chain. This is due to a concept known as energy transfer efficiency, which states that only about 10% of the energy is transferred from one trophic level to the next. The rest of the energy is lost as heat or used for essential life processes.

This decrease in available energy limits the number of trophic levels in a food chain. As energy is lost with each transfer, there is not enough energy to sustain higher levels. This is why food chains typically have a limited number of trophic levels, usually ranging from three to five.

To summarize, a decrease in available energy at higher trophic levels is what restricts the number of trophic levels in a food chain.