The correct answer is (d). Habitat destruction is the main reason for the decrease in biodiversity. Habitat destruction is the permanent removal of a natural habitat, usually to make room for something new, such as a farm, city, or road. When a habitat is destroyed, the plants and animals that live there are forced to find a new home or die. Habitat destruction is a major threat to biodiversity because it destroys the homes of many different species. It can also lead to the extinction of species if they are unable to find a new home.
(a) Habitat pollution is the contamination of a natural habitat with harmful substances. This can be caused by human activities, such as industrial pollution or the use of pesticides. Habitat pollution can harm plants and animals, and can make it difficult for them to survive. However, habitat pollution is not the main reason for the decrease in biodiversity.
(b) Introduction of exotic species is the introduction of a species to an area where it did not previously exist. This can happen accidentally, such as when a ship carrying cargo from one country to another accidentally releases an invasive species. It can also happen intentionally, such as when a species is introduced to an area in an attempt to control pests. The introduction of exotic species can have a negative impact on biodiversity, as the introduced species can compete with native species for resources, or even prey on them. However, the introduction of exotic species is not the main reason for the decrease in biodiversity.
(c) Over-exploitation is the use of a resource at a rate that is greater than the rate at which it can be replenished. This can lead to the depletion of the resource, and can also have a negative impact on the biodiversity of the area. For example, over-fishing can lead to the decline of fish populations, which can then have a knock-on effect on the animals that rely on fish for food. However, over-exploitation is not the main reason for the decrease in biodiversity.