The organisms that are best represented in the upper and middle reaches in low salinities (5–30 ppt):

Correct option is B

Correct Answer: (b) True estuarine organisms
Explanation:
→ True estuarine organisms are adapted to variable salinity, especially mid-estuarine salinity ranges of 5–30 ppt.
→ They are physiologically equipped to tolerate daily and seasonal fluctuations.
Information Booster:
→ These organisms include certain crustaceans, molluscs and estuarine fish species.
Additional Information:
(a) Oligohaline species prefer 0.5–5 ppt.
(c) Euryhaline marine species tolerate wide range but prefer higher salinity.
(d) Stenohaline marine species cannot tolerate low salinity.