The correct answer is option 4: rainfall is high. Salinity in ocean water decreases when there is high rainfall.
Option 1: Evaporation is more: While it is true that evaporation can increase the salinity of ocean water by removing freshwater and leaving behind salt, this does not cause the salinity to decrease.
Option 2: Wind velocity is high: Wind velocity does not directly affect the salinity of ocean water.
Option 3: Humidity is high: Humidity, or the amount of moisture in the air, does not directly impact the salinity of ocean water.
Option 4: Rainfall is high: High rainfall can dilute the saltwater in the ocean and lower its salinity. Rainwater is freshwater and when it mixes with the ocean, it reduces the concentration of salt, leading to a decrease in salinity.
Overall, option 4 is the correct answer as it accurately explains how salinity decreases in ocean water.