Upper reaches of estuary are:

Correct option is B

Correct Answer: (b) Oligohaline
Explanation:
→ The upper reaches of an estuary, where freshwater influence is strongest, typically fall in the oligohaline salinity range (~0.5–5 ppt).
→ Organisms here are adapted to low salinity and variable conditions.
Information Booster:
→ Salinity zones: freshwater/oligohaline (upper), mesohaline (mid), polyhaline/euhaline (lower/near sea).
Additional Information (Other Options):
(a) Limnetic: freshwater lake zone; (c) mesohaline: mid-estuary (5–18 ppt); (d) euhaline: near full seawater salinity.