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