Which of the following is a major river in Bangladesh that also forms the main distributary of the Ganga river system?

Introduction / Context:
The great Ganga river does not end suddenly at the Indian border. Instead, as it enters Bangladesh, it divides into distributaries that spread across the delta before meeting the Bay of Bengal. For competitive exams, one common question is to identify the main distributary of the Ganga in Bangladesh and to connect it with the name by which it is known there. This not only tests river system knowledge but also familiarity with cross border geography in South Asia.

Given Data / Assumptions:

The river must be a major river in Bangladesh.
It must also be the main distributary of the Ganga as it enters Bangladesh.
Options include Gandak, Kosi, Gomati and Padma.
We assume standard naming conventions where the Ganga is called by a different name after entering Bangladesh.

Concept / Approach:
As the Ganga reaches the border of India and Bangladesh, near the area of Farakka and beyond, it branches and the main course inside Bangladesh is called the Padma. The rivers Gandak and Kosi are important tributaries that join the Ganga in India from the north before the river reaches Bangladesh, while Gomati is a smaller river mainly in India. Therefore, the correct approach is to recall that the Ganga is renamed Padma in Bangladesh, making Padma the major distributary being asked about.

Step-by-Step Solution:
Step 1: Recall that the Ganga flows from the Himalayas across northern India and then enters Bangladesh.Step 2: Remember that once the main stream enters Bangladesh, it is locally known as the Padma.Step 3: Note that Gandak and Kosi are tributaries joining the Ganga in Bihar and eastern Uttar Pradesh, not distributaries in Bangladesh.Step 4: Eliminate the incorrect rivers and select Padma as the main distributary of the Ganga in Bangladesh.

Verification / Alternative check:
You can cross check using a political and physical map of India and Bangladesh. The Ganga, after passing through West Bengal, turns south east and enters Bangladesh, where maps label the main channel as the Padma. From there it later meets the Jamuna (the name for the Brahmaputra in Bangladesh) and downstream segments may be labelled the Meghna. No standard atlas labels Gandak, Kosi or Gomati as the primary distributary of the Ganga in Bangladesh, so this independent check confirms that Padma is correct.

Why Other Options Are Wrong:
Gandak is a Himalayan river that flows through Nepal and Bihar and joins the Ganga as a tributary, not as a distributary in Bangladesh.
Kosi is another Himalayan tributary that meets the Ganga in Bihar and is famous for floods in the Indian state of Bihar, not for being the main distributary in Bangladesh.
Gomati is a river flowing mainly in Uttar Pradesh and does not function as the main distributary of the Ganga in the Bangladesh delta.
Hence these options do not satisfy the condition of being a major river in Bangladesh that is the main distributary of the Ganga.

Common Pitfalls:
Students often confuse the roles of tributaries and distributaries, or assume that any well known northern river could be part of the delta distributary network. Another frequent confusion is mixing up the local names of rivers as they cross national borders. To avoid such mistakes, remember that tributaries bring water into the main river upstream, while distributaries branch out as the river approaches its mouth, as in a delta. Also, fix in memory that the Ganga becomes the Padma in Bangladesh.

Final Answer:
The major river in Bangladesh that is the main distributary of the Ganga is the Padma.