Which one of the following is not a lagoon?

Answer: (c) Periyar lake

Periyar lake is a reservoir formed by the Mullaperiyar dam, not a lagoon.

The idea

A lagoon is a coastal water body separated from the open sea by a barrier such as a sand spit. It may retain a narrow sea connection and contain brackish water. A reservoir instead forms when a dam impounds a river, even if people call it a lake.

Waves and currents move coastal sand → A barrier or spit grows → Water is partly enclosed behind it → Sea exchange continues through openings → Freshwater and seawater mix → A lagoon develops

Why the others are wrong

(a) Ashtamudi is a Kerala backwater lagoon.

(b) Chilika is India’s largest lagoon.

(d) Pulicat is a lagoon.

Elimination technique

An inland dam-created water body fails the defining coastal-barrier test for a lagoon. That identifies Periyar without relying on whether its name contains “lake”.

Also know

Vembanad belongs to Kerala’s backwater system. Pulicat straddles Andhra Pradesh and Tamil Nadu, with Sriharikota forming its seaward barrier. Freshwater Kolleru lies between two deltas but is not the same type as Chilika. Mullaperiyar created the Periyar reservoir in an inland hill setting.