Final answer:
To help reduce water pollution when disposing of household chemicals such as oil-based paint, it is best to take them to a hazardous waste collection site.
Step-by-step explanation:
To help reduce water pollution when disposing of household chemicals such as oil-based paint, the best option is to take them to a hazardous waste collection site (option c). This ensures that the chemicals are handled and disposed of properly to prevent contamination of water bodies. Dumping them over a hill (option a) or pouring them down the kitchen drain (option b) can lead to the chemicals entering the water supply and causing pollution.