ANS:C - Oligotrophic lakes

Option (C) is correct.

A perfect ecological system means that are producers, consumers and decomposers present in the system in such a manner that food and energy flows from one level to the other. The pollutants (nitrogen and phosphorous) disturb this equilibrium by increasing productivity of a lake increases productivity of a lake. As the productivity of a lake increases, its water quality reduces and equilibrium is disturbed. Oligotrophic lakes have low level of productivity so they can be considered to be perfect ecological system.