All waters in nature, whether rain water or ocean water, contain dissolved mineral salts. Salinity is the term used to define the total content of dissolved salts in sea water. It is calculated as the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater. It is usually expressed as parts per thousand (o/oo) or ppt. Salinity is an important property of sea water. Salinity of 24.7 o/oo has been considered as the upper limit to demarcate ‘brackish water’. In the land locked Red Sea, salinity is as high as 41o/oo, while in the estuaries and the Arctic, the salinity fluctuates from 0 – 35 o/oo, seasonally.
The salinity variation in the Pacific Ocean is mainly due to its shape and larger areal extent. The highest salinity is recorded between 15° and 20° latitudes. Maximum salinity (37 o/oo) is observed between 20° N and 30° N and 20° W – 60° W. It gradually decreases towards the north.

1, 3 and 4 are correct.

The principal tributaries of Brahmaputra River joining from right are the Lohit, the Dibang, the Subansiri, the Jiabharali, the Dhansiri, the Manas, the Torsa, the Sankosh and the Teesta whereas the Burhidihing, the Desang, the Dikhow, the Dhansiri and the Kopili joins it from left.
Hindon and Sindh are the tributaries of Yamuna River.

Situated on the northeastern tip of the country, the state of Arunachal Pradesh is a part of Eastern Himalayan Ranges. The important forests types found in the state are Tropical evergreen, semi evergreen, deciduous, Pine, Temperate, Alpine and grassland.

For rice, high rainfall, temperature and clayey soil is required which can hold water. For Cotton, light rainfall is needed. For Jute high rainfall and temperature is needed.
Wheat requires moderate temperature and rainfall during growing season and bright sunshine at the time of harvest. It thrives best in well drained loamy soil.
Maize requires moderate temperature, rainfall and lots of sunshine. It needs well-drained fertile soils.