The correct answer is excess irrigation.

Soil salinization, also known as soil salinity, is the accumulation of soluble salts in the soil to the extent that it adversely affects plant growth. This occurs when the salts are not leached sufficiently by rainfall or drainage, and the salt concentration gradually increases to harmful levels.

Excess irrigation plays a crucial role in this process. When more water than necessary is used for irrigation, the excess water evaporates, leaving behind the dissolved salts concentrated in the topsoil. This process, especially in arid and semi-arid regions with poor drainage, leads to the gradual buildup of salts, rendering the soil infertile and unsuitable for agriculture.

Pesticides, soil erosion, and crop rotation do not directly cause soil salinization. While soil erosion can indirectly impact soil salinity by altering soil structure and drainage, it's not the primary cause. Pesticides primarily affect the biological aspects of the soil, and crop rotation is a soil management technique intended to improve soil health and fertility.

In contrast, excess irrigation directly contributes to the accumulation of salts at the soil surface, making it the most significant factor in causing soil salinization.