The Sundarbans delta, the largest delta in the world, has been formed by the rivers:

The Sundarbans delta, also called the Ganga-Brahmaputra delta or the Bengal delta, has been formed by the sediments deposited by the Ganga, the Brahmaputra and the Meghna rivers. It is the largest delta in the world, covering about 1 lakh square km in India and Bangladesh.

Formation:

• These rivers carry enormous amounts of silt from the Himalayas.
• As they slow down near the Bay of Bengal, they deposit the silt, building up land and forming many channels, creeks and islands.
• The delta is still growing in some areas and being eroded in others.

The Sundarbans:

• The southern, coastal part of the delta is covered by the Sundarbans, the largest mangrove forest in the world, about 10,000 square km, shared by India (about 40 percent) and Bangladesh (about 60 percent).
• The name is believed to come from the "sundari" tree (Heritiera fomes), a common mangrove species there.

UNESCO and Ramsar status:

• The Sundarbans National Park in India was declared a UNESCO World Heritage Site in 1987, and the Bangladesh part in 1997.
• The Indian Sundarbans is also a Biosphere Reserve and a Ramsar site (2019), one of the largest in India.

Wildlife:

The Sundarbans is the only mangrove forest in the world with tigers. The Royal Bengal tigers here are known for swimming and for their adaptation to a salty, tidal environment.

Other animals include the saltwater crocodile, the Gangetic dolphin, the olive ridley turtle and the fishing cat.

Threats:

Rising sea levels, cyclones (such as Aila in 2009 and Amphan in 2020), increasing salinity, erosion and human pressure threaten the delta and its people.

Why not the other options?

• ✗The Indus and Sutlej flow into the Arabian Sea through Pakistan.
• ✗The Narmada and Tapi flow west into the Arabian Sea and do not build large deltas.
• ✗The Mahanadi and Brahmani form deltas in Odisha.