Answer:
(B) the mouth
Explanation:
"Light penetration in a river or stream is likely to be lowest at the mouth of the river".
This is because in the beginning, the river is basically free of particulate matter and is also clear and so there is an ample amount of light penetration in the beginning.
At the middle portion, the suspended thing is high, but due to the speed of the water, the particles get cleared.
However, at the mouth of the river, there is a large concentration of particulate matter, and thus light penetration is the lowest.
Hence, option B is correct.
