Final answer:
Option d is the correct answer. The green glands in freshwater crayfish help control the internal water balance by flushing out excess water through excretion, maintaining osmoregulation despite the hypotonic environment they inhabit.
Step-by-step explanation:
The green glands in the freshwater crayfish act as an effective "flood control" device by d. Flushing out excess water through excretion. Freshwater crayfish must regulate the water that enters their bodies due to the hypotonic environment they live in. By excreting dilute urine, they manage to maintain the appropriate solute concentrations within their bodies and prevent their cells from taking in too much water through osmosis. This process is part of their osmoregulatory mechanisms, which are crucial for survival in freshwater habitats. Unlike mechanisms of nitrogenous waste excretion observed in some arthropods and annelids, where metabolic wastes like uric acid are excreted as thick paste or powder minimizing water loss, crayfish excrete more dilute urine, which helps them regulate their internal water balance.