The correct answer is: C. Cement concrete sewers.
Cement concrete sewers have the highest non-scouring velocity because they are the most durable and have the smoothest surface. This means that they are less likely to be damaged by scouring, which is the process of erosion caused by the movement of water and sediment.
Glazed bricks sewers are also durable, but they have a rougher surface than cement concrete sewers. This means that they are more likely to be damaged by scouring.
Cast iron sewers are not as durable as cement concrete or glazed bricks sewers, and they have a very rough surface. This means that they are the most likely to be damaged by scouring.
Stone ware sewers are durable and have a smooth surface, but they are not as common as cement concrete sewers.
In conclusion, cement concrete sewers have the highest non-scouring velocity because they are the most durable and have the smoothest surface.