The average salinity of the open ocean, including the Indian Ocean, is about:

The average salinity of the world's oceans is about 35 parts per thousand, which means that 1,000 grams of sea water contain about 35 grams of dissolved salts. The open Indian Ocean has an average salinity close to this value.

Salinity in parts of the Indian Ocean:

• The Arabian Sea has higher salinity (about 36 or more), because of high evaporation and low fresh-water inflow.
• The Bay of Bengal has lower salinity (about 30 to 34, and even less near river mouths), because rivers such as the Ganga, Brahmaputra, Godavari and Krishna bring huge amounts of fresh water.
• The Red Sea and the Persian Gulf, connected to the Indian Ocean, have very high salinity (about 40 or more).
• Main salts: sodium chloride (common salt) makes up about 78 percent of the dissolved salts, followed by magnesium chloride and magnesium sulphate.

Where the salts come from:

• Weathering of rocks on land releases minerals that rivers carry to the sea.
• Gases and minerals released by underwater volcanoes and hydrothermal vents add salts.
• As water evaporates from the sea, the salts remain behind.

Highest and lowest salinity:

• The Dead Sea (about 340 parts per thousand) and Lake Van in Turkey (about 330) are among the saltiest water bodies on Earth.
• The Baltic Sea has very low salinity (about 7 to 8).

Salinity and density:

• Water with higher salinity is denser and tends to sink, while water with lower salinity floats above it.
• This affects deep ocean circulation.

About the Indian Ocean:

• It is the third largest ocean, covering about 20 percent of the Earth's water surface.
• It is the only ocean named after a country, India.
• It is bounded by Asia to the north, Africa to the west and Australia to the east.

Why not the other options?

• ✗3.5 parts per thousand is ten times too low.
• ✗100 and 350 parts per thousand are far too high for the open ocean; values like 340 are found only in hypersaline lakes such as the Dead Sea.