Scientists have determined the average salinity of ocean water to be equal

The correct answer is 35 g/kg.  

The amount of dissolved salt in a given volume of water is known as salinity. Salinity is either articulated in the form of parts per thousand or in the form of grams of salt per kilogram of water. On average, the world's oceans seawater exhibits a salinity of about 3.5 percent or 35 parts per thousand. This signifies that for every 1000 ml or 1 liter of seawater, there are 35 grams of salt dissolved in it.  

The density of water is 1 kg per liter or 1000 g per 1000 ml

Thus, 3.5 % of it would be: 3.5 % × 1000 = 35 g/kg or 35 g/L

a rain shadow is a dry region of land on the side that is protected against the prevailing winds of a mountain range. the prevailing winds are the winds that occur most of the time in a particular place of the earth

prevailing winds carry air toward the mountain range. as the air rises up over a mountain range, the air cools, water vapor condenses, and clouds form. on this side of the mountains, called the windward side, precipitation falls in the form of  rain  or snow. this dry air produces a  rain shadow.