ANS:D - all the above.

In the case of sewer lines, all of the mentioned tests are commonly carried out for different purposes:

1. Water test to check water tightness of the joints: This test involves filling the sewer line with water and observing whether there are any leaks at the joints. It ensures that the joints are properly sealed to prevent leakage of sewage or infiltration of groundwater.
2. Test for straightness with the help of a lamp and mirror: This test is performed to check the alignment or straightness of the sewer line. A lamp is placed at one end of the sewer line, and a mirror is placed at the other end. If the reflected light from the lamp is seen in the mirror without any distortion, it indicates that the sewer line is straight.
3. Obstruction test with the help of a smooth ball of diameter 13 mm: This test involves passing a smooth ball through the sewer line to check for any obstructions or blockages. The diameter of the ball (13 mm) is typically chosen to ensure that it can pass through the sewer line without getting stuck. If the ball encounters any obstructions, it indicates the presence of blockages that need to be addressed.