salinity is the correct answer

Explanation:

Seawater is toxic to humans because your body is unable to get rid of the salt that comes from seawater. Your body's kidneys normally remove excess salt by producing urine, but the body needs freshwater to dilute the salt in your body for the kidneys to work properly. Normally that is not a problem, as you are always drinking water and eating food with water. Tissue in your body also contains freshwater that can be used. But if there is too much salt in your body, your kidneys cannot get enough freshwater to dilute the salt and your body will fail. 

(b) salinity