Ans.
The large dams show various environmental consequences, which affect chemical, physical, and biological properties of the river and nearby environment.
Dam leads to transformation to a artificial slack-water reservoir ecosystem from a free-flowing river ecosystem. Change in chemical composition, climate, and chemical composition of rivers are generally not suitable to the aquatic organisms, such as fishes and leads to decrease in population of those organisms.
Thus, the correct answer is option C) as 'fishes would decrease due to the change in environment.'