Biological equilibrium is an equilibrium between

$\begin{array}{1 1}(a)\;\text{Producers & decomposers}\\(b)\;\text{Producers & consumers}\\(c)\;\text{Producers,consumers & decomposers}\\(d)\;\text{Consumers & decomposers}\end{array}$

Biological equilibrium is an equilibrium between producers,consumers & decomposers
Hence (c) is the correct answer.