The one of the following that is used to connect two pipes together in a straight line is a:

The connection of two pipes in a straight line is commonly accomplished using a union. A union is a type of fitting designed to join two segments of pipe while allowing for disassembly without having to cut the pipe. This feature is particularly important in sewage treatment and plumbing applications, where maintenance and repairs may require the removal of pipe sections without damaging them. Unions facilitate easy assembly and disassembly, which is essential in conditions where regular access to the piping system is necessary for inspections or repairs.

Other options, while they may relate to piping, serve different functions. A manifold typically refers to a component that distributes or collects flow from several pipes, not connecting two pipes in a straight line. A divider is not a standard term used in plumbing for connecting pipes and might refer to a device that separates or directs flows but does not connect. A band could imply a clamping device, but it doesn't perform the same function as a union in terms of providing a removable and secure joint between two pipes.