Shyok, Gilgit and Zanskar are major tributaries of the river:

The Shyok, Gilgit and Zanskar (Zaskar) are important tributaries of the Indus, which drains the high mountain regions of Ladakh, Gilgit-Baltistan and Tibet.

Shyok:

• It rises from the Rimo glacier in the Karakoram, in the north-eastern part of Ladakh.
• It flows south-east, then makes a sharp U-turn and flows north-west, joining the Indus in Gilgit-Baltistan near Skardu.
• Its main tributary is the Nubra river.
• The Shyok has historically caused floods when glacial ice dams burst; its name is said to mean "river of death".

Zanskar:

• It rises in the Zanskar region of Ladakh and flows north to join the Indus near Nimmu, about 35 km from Leh.
• The meeting of the clear, green Indus and the muddy Zanskar at Nimmu is a famous sight.
• In winter, the frozen Zanskar becomes the "Chadar Trek", a route walked on ice.

Gilgit:

• It flows through the Gilgit-Baltistan region (under Pakistan's occupation) and joins the Indus near Gilgit.
• The Hunza river is its tributary; the Karakoram Highway runs along the Hunza valley to China.

Other tributaries of the Indus:

• Right-bank tributaries: Shyok, Gilgit, Hunza, Swat, Kurram, Tochi, Gomal and Zhob.
• Left-bank tributaries: Zanskar, Suru, Dras, and the five great rivers of Punjab (Jhelum, Chenab, Ravi, Beas and Sutlej), which join the Indus through the Panjnad.

The Indus in Ladakh:

• The Indus flows through Ladakh between the Ladakh Range and the Zanskar Range, past Leh.
• Its upper valley is a cold desert with very little rainfall.

Why not the other options?

• ✗The Ganga and Brahmaputra drain the central and eastern Himalayas.
• ✗The Yamuna is a tributary of the Ganga.