The correct answer is D. > 1.5 m.
The spacing of manholes along a straight portion of a sewer is typically determined by the diameter of the sewer, the type of soil, and the slope of the sewer. In general, the smaller the diameter of the sewer, the closer the manholes should be spaced. This is because smaller sewers are more likely to clog, and it is important to be able to access them easily for maintenance. The type of soil also affects the spacing of manholes. In loose soils, manholes should be spaced closer together to prevent the sewer from collapsing. The slope of the sewer also affects the spacing of manholes. In steeper sewers, manholes should be spaced closer together to prevent sediment from settling in the sewer.
In this case, the spacing of manholes is 300 m. This indicates that the diameter of the sewer is relatively large. This is because larger sewers can be spaced further apart without the risk of clogging or collapse. The type of soil and the slope of the sewer are not given, so it is not possible to say for certain what the diameter of the sewer is. However, it is likely that the diameter is greater than 1.5 m.