Explanation:

A gully trap is a type of plumbing fixture that is installed at the junction of an unfoul house drain and a foul house drain. It is designed to prevent foul odors and gases from the sewer system from entering the house.

Here is a detailed explanation of each option:

a) A house sewer and a municipal sewer: A gully trap is not provided at the junction of a house sewer and a municipal sewer. A house sewer is a pipe that carries wastewater from a house to the municipal sewer system. Since the municipal sewer system is designed to handle foul water, there is no need for a gully trap at this junction.

b) An unfoul house drain and a foul house drain: This is the correct option. A gully trap is provided at the junction of an unfoul house drain and a foul house drain. An unfoul house drain carries wastewater from fixtures that do not produce foul water, such as sinks and showers. A foul house drain, on the other hand, carries wastewater from fixtures that produce foul water, such as toilets. The gully trap is installed at this junction to prevent foul odors and gases from entering the house.

c) Any two house drains: A gully trap is not provided at the junction of any two house drains. The purpose of a gully trap is to prevent foul odors and gases from the sewer system from entering the house. If there are no foul drains involved, there is no need for a gully trap.

d) None of these: The correct answer is option B, as explained above.

In summary, a gully trap is provided at the junction of an unfoul house drain and a foul house drain. It is not provided at the junction of a house sewer and a municipal sewer or any two house drains. The purpose of a gully trap is to prevent foul odors and gases from the sewer system from entering the house.