Answer:
Salinity decreases because the stream adds fresh water to the ocean.
Step-by-step explanation:
Fresh water is called the water of rivers, lakes and most underground water, with a salinity close to zero, as opposed to sea water (which generally has a salinity close to 35 grams of dissolved salts per liter). In this case, we can say that if fresh water were added to sea water, the salinity of that water will be diluted and consequently decrease.
For this reason, we can say that the correct answer to your question is "Salinity decreases because the stream adds freshwater to the ocean."