The entry of foul - smelling gases into the house coming from the sewers can be prevented by

Explanation:

To prevent the entry of foul-smelling gases from the sewers into the house, the most effective methods involve using water seals and vent pipes. Let's analyze the options provided:

1. Exhaust fan: While an exhaust fan can help ventilate a space and remove odors, it does not specifically prevent the entry of gases from the sewer system into the house. It may help mitigate odors once they are inside but is not a preventative measure for sewer gases.

2. Providing water seals for all the fixtures: Water seals, such as those found in P-traps and S-traps, act as barriers by holding a small amount of water, which prevents sewer gases from traveling back up through the plumbing into the house. While effective, relying solely on water seals might not be sufficient in cases where water evaporates or the traps dry out.

3. Providing water seals for all the fixtures and vent pipe in the plumbing system: This option is the most effective and is considered the standard practice in plumbing design. Water seals prevent gases from entering through fixtures, while vent pipes ensure that sewer gases are safely vented outside the house. The vent pipes also help maintain atmospheric pressure in the plumbing system, preventing siphoning of water from the traps.

4. Providing sufficient vent pipes in the plumbing system: Vent pipes are essential in allowing gases to escape and preventing the build-up of pressure in the plumbing system. While vent pipes are necessary, providing them without water seals would not prevent gas entry through fixtures if water traps dry out.