Water pollution is a significant environmental issue that occurs when pollutants contaminate water bodies, such as rivers, lakes, and oceans. These pollutants can have detrimental effects on aquatic ecosystems, human health, and the overall quality of water resources. Several factors contribute to water pollution, including sewage, industrial waste, and the use of fertilizers and pesticides.

a) Sewage:
- Sewage refers to the waste materials, primarily human excreta and wastewater, that are discharged from homes, industries, and other sources into water bodies.
- When sewage is released into water without proper treatment, it introduces harmful bacteria, viruses, and pathogens that can cause waterborne diseases.
- Sewage also contains high levels of nutrients, such as nitrogen and phosphorus, which can lead to excessive growth of algae and other aquatic plants, causing eutrophication. This disrupts the balance of the ecosystem and depletes oxygen levels, leading to fish kills and other negative impacts on aquatic life.

b) Industrial waste:
- Industrial activities produce a wide range of waste materials, including chemicals, heavy metals, and toxic substances, which can contaminate water bodies if not properly managed.
- Untreated or poorly treated industrial waste can contain harmful pollutants such as heavy metals (e.g., mercury, lead), organic compounds (e.g., solvents, pesticides), and radioactive materials.
- These pollutants can have severe ecological and health impacts. For example, heavy metals can accumulate in aquatic organisms, disrupting their reproductive systems and causing other health issues. Organic compounds can be toxic to aquatic life and persist in the environment for long periods, leading to bioaccumulation.

c) Fertilizers and pesticides:
- The use of fertilizers and pesticides in agriculture is another significant contributor to water pollution.
- Fertilizers contain high levels of nutrients, such as nitrogen and phosphorus. When applied in excess or during heavy rainfall, these nutrients can be washed off into nearby water bodies, causing eutrophication.
- Pesticides, which are chemicals used to control pests and diseases in crops, can also be washed off into water bodies during rainfall or irrigation, contaminating the water.
- Pesticides can be toxic to aquatic organisms, including fish, amphibians, and invertebrates, and can disrupt the balance of the aquatic ecosystem.

d) All of these:
- All the mentioned factors contribute to water pollution, either individually or in combination.
- Sewage, industrial waste, and the use of fertilizers and pesticides are major sources of water pollution globally.
- To mitigate water pollution, it is crucial to implement proper wastewater treatment systems, regulate industrial activities, and promote sustainable agricultural practices that minimize the use of fertilizers and pesticides.

In summary, water pollution is caused by various factors, including sewage, industrial waste, and the use of fertilizers and pesticides. These pollutants can have severe ecological and health impacts, highlighting the importance of managing and reducing their release into water bodies.