Salinity in ocean water decreases when :

Ocean salinity is primarily determined by the balance between evaporation and the addition of freshwater. Salinity decreases when freshwater is added to the ocean, which dilutes the salt concentration [c1][c5]. High rainfall (precipitation) is a major source of freshwater input that significantly reduces surface salinity [c2][t4]. For instance, equatorial waters exhibit lower salinity than the global average due to heavy rainfall and high relative humidity [c2]. Conversely, processes that remove freshwater, such as high evaporation rates or ice formation, lead to an increase in salinity [t1][t4]. While wind and ocean currents influence salinity by redistributing water masses, they do not inherently decrease salinity unless they transport lower-salinity water into a region [c1][c3]. Therefore, high rainfall is the most direct factor among the options that causes a decrease in ocean salinity [t6][t9].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 33: Ocean temperature and salinity > Factors Affecting Ocean Salinity > p. 518
2. [2] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 12: The Oceans > Salinity of the Ocean > p. 107
3. [3] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 12: Water (Oceans) > Vertical Distribution of Salinity > p. 106
4. [4] https://www.britannica.com/science/seawater/Salinity-distribution