The condition that indicates a polluted environment is:

Step-by-step solution

(2) Clean water would have B.O.D value of less than 5 ppm whereas highly polluted water coluld have a B.O.D value of 17 ppm or more. (3) The process in which nutrient enriched water bodies support a dense plant population which kill animal life by depriving it of oxygen results in subsequent loss of biodiversity is known as Eutrophication. (4) If the concentration of dissolved oxygen of water is below 6 ppm, the growth of fish get inhibited.