Which is the longest river flowing predominantly through the Indian state of Odisha?

Introduction / Context:
This question tests knowledge of state-wise river systems, focusing on Odisha. Rivers play a central role in the agriculture, power generation and culture of a state. For Odisha, the Mahanadi river system is particularly important, and many exams expect candidates to know which river is the longest within the state's territory.

Given Data / Assumptions:
- The question asks about the longest river in Odisha, not necessarily the longest river of India.
- The options include Mahanadi, Brahmani, Baitarani, Subarnarekha and Rushikulya, all familiar east-flowing rivers in eastern India.
- Length is considered with respect to the river system that significantly drains Odisha and has major impact in the state.
- We use standard Indian geography references.

Concept / Approach:
The Mahanadi is a major east-flowing peninsular river that rises in Chhattisgarh and flows through Odisha before meeting the Bay of Bengal. It has a large catchment area within Odisha and forms an extensive delta near Cuttack and Paradeep. Among the rivers listed, Mahanadi is the most voluminous and longest in terms of its course relevant to Odisha. The other rivers like Brahmani, Baitarani, Subarnarekha and Rushikulya are important but shorter or have smaller drainage areas in comparison.

Step-by-Step Solution:
Step 1: Recall the major rivers of Odisha: Mahanadi, Brahmani, Baitarani, Subarnarekha and a few smaller coastal rivers like Rushikulya.Step 2: Among these, identify which one has the longest overall course and the largest basin associated with Odisha.Step 3: Mahanadi flows from central India through Odisha and has a long course before forming a large delta and entering the Bay of Bengal.Step 4: Compare with Brahmani and Baitarani, which are significant but shorter and have relatively smaller basins inside Odisha.Step 5: Subarnarekha and Rushikulya are even smaller in total length and influence within the state.Step 6: Therefore, the longest river associated with Odisha is Mahanadi.

Verification / Alternative check:
Standard geography textbooks and exam guides explicitly state that "Mahanadi is the longest river of Odisha." Atlases showing river lengths and basin areas also confirm that Mahanadi dominates the river network of the state. Major irrigation and hydropower projects like the Hirakud Dam are built on the Mahanadi, underlining its importance and length compared to other rivers in the region.

Why Other Options Are Wrong:
Brahmani: An important east-flowing river formed by the combined flow of the Sankh and South Koel, but shorter than Mahanadi in the context of Odisha.

Baitarani: A smaller river flowing through northern Odisha and Jharkhand, not the longest in the state.
Subarnarekha: A relatively shorter river flowing through Jharkhand, West Bengal and a small part of Odisha.
Rushikulya: A coastal river entirely within Odisha but much shorter in length than Mahanadi.

Common Pitfalls:
Students may confuse "most important" with "longest" or may recall multiple exam statements about Brahmani or Baitarani and assume one of them is the correct choice. Another common mistake is to think that a river entirely within a state, like Rushikulya, must be the longest there. It is important to differentiate between length, basin area and economic importance. In terms of length and basin dominance for Odisha, Mahanadi clearly stands out.

Final Answer:
Mahanadi