The correct answer is A. Traps are water seals which prevent the entry of foul gases.
A trap is a device that is used to prevent the entry of foul gases from sewers into buildings. It does this by creating a water seal, which is a barrier of water that blocks the passage of gases. The water seal is maintained by the trap being filled with water, which is prevented from draining out by a U-shaped bend in the pipe.
Option B is incorrect because traps are not used to trap rats. Rats can enter sewers through small openings, and traps will not prevent them from doing so.
Option C is incorrect because traps do not dissolve foul gases. Foul gases are prevented from entering buildings by the water seal in the trap.
Option D is incorrect because traps do not create symphonic action to increase the quick disposal of sewerage. Traps are designed to prevent the entry of foul gases, not to increase the speed of sewerage disposal.