During eutrophication, there is a natural process where excessive nutrients, such as nitrogen and phosphorous, enter the water bodies like lakes. These nutrients come from various sources like agricultural runoff, sewage discharge, and industrial effluents.

The correct answer is option 4, which states "Excessive entry of nutrients into water." Eutrophication occurs when there is an excessive influx of nutrients into the lake, promoting the growth of algae and other aquatic plants.

As these plants proliferate, they form dense layers on the surface of the lake, creating a phenomenon known as algal blooms. These blooms can block sunlight from reaching the deeper layers of the lake, leading to a decrease in dissolved oxygen levels.

Option 1, "Rapid destruction of algal growth," is incorrect. Eutrophication actually promotes the growth of algae rather than destroying it.

Option 2, "Excessive availability of dissolved oxygen," is also incorrect. Eutrophication leads to a decrease in dissolved oxygen due to the depletion caused by the algal blooms.

Option 3, "Loss of dissolved nutrients from water," is also incorrect. Eutrophication involves an increase in the amount of nutrients in the water rather than a loss.

Therefore,