When a meander loop of a river is cut off from the main channel, it forms a cut off lake that is also called an ________ lake.

Introduction / Context:
Rivers in their mature and old stages often flow across broad floodplains and develop winding curves known as meanders. Over time, these meanders can change shape and even get cut off from the main river channel, forming distinct features on the landscape. Geography questions frequently ask about the specific name given to the lake formed when a meander loop is cut off. This helps students link fluvial processes with landforms.

Given Data / Assumptions:

The question mentions a meander loop that becomes cut off from the main river channel.
It states that the resulting water body is called a cut off lake and asks for the specific term for such a lake.
Options provided are ox-bow, oasis, lagoon, and tectonic.
We assume basic knowledge of river processes and types of lakes in physical geography.

Concept / Approach:
When a river meander becomes very pronounced, the neck of the meander can gradually narrow due to erosion on the outer banks and deposition on the inner banks. During floods, the river may take a shorter course across the neck, abandoning the old loop. The abandoned loop fills with water and becomes a crescent shaped lake known as an ox bow lake. An oasis is a water source in a desert, a lagoon is a shallow water body separated from the sea, and tectonic lakes form due to crustal movements. Only ox bow matches the description of a cut off meander lake.

Step-by-Step Solution:
Step 1: Identify that the question deals with river meanders on a floodplain. Step 2: Recall that with time, erosion and deposition modify meanders and can isolate a loop from the main channel. Step 3: Understand that once the river adopts a straighter course and abandons the old loop, the isolated loop forms a lake. Step 4: Remember that this crescent shaped water body is termed an ox bow lake in geography. Step 5: Compare with other options and see that oasis, lagoon, and tectonic do not describe this river formed lake, confirming ox-bow as the correct choice.

Verification / Alternative check:
To verify, visualise diagrams from textbooks showing a series of meanders and the formation of an ox bow lake. The shape resembles the bow of an ox yoke, which is why it has that name. Satellite images of large rivers like the Ganga or Mississippi often show many ox bow lakes dotting the floodplain. None of these images or diagrams label such features as oases, lagoons, or tectonic lakes, which confirms that ox bow is the correct term.

Why Other Options Are Wrong:
Oasis is wrong because it refers to a fertile spot in a desert where water is available, usually from groundwater, not a former river meander. Lagoon is wrong because it describes a shallow coastal water body separated from the open sea by a barrier such as a sandbar or coral reef. Tectonic is wrong because tectonic lakes form due to movements of the Earth crust, such as faulting or subsidence, and are not directly related to meandering rivers. These definitions do not match the process described in the question.

Common Pitfalls:
Some students may confuse ox bow lakes with lagoons because both can appear curved or crescent shaped in some diagrams. Others may vaguely remember the word oasis and select it without thinking about the desert context. To avoid mistakes, always link the term ox bow specifically with cut off meanders in river floodplains. Using labelled diagrams while revising fluvial landforms is an effective way to retain these concepts.

Final Answer:
A cut off meander loop forms an ox-bow lake.