The correct answer for the question is option 4, "Population of rats increases." Bamboo flowering triggers a boom in the rat population, as bamboo seeds provide a plentiful food source for rats. This is commonly known as a rat or rodent plague.

Option 1, "Rainfall decreases," is not directly caused by bamboo flowering. Bamboo flowering may have an indirect effect on the ecosystem, but it does not directly influence rainfall patterns.

Option 2, "Pests and insects increase," is not specifically linked to bamboo flowering. While an increase in rats could lead to an increase in certain pests that prey on them, it is not a direct consequence of bamboo flowering.

Option 3, "Land becomes infertile," is unrelated to bamboo flowering. Bamboo flowering primarily affects the bamboo population and the ecosystem around it, but it does not render the land infertile.

In conclusion, the correct answer is option 4, as bamboo flowering leads to an increased population of rats.