According to the MUTCD, what does a single solid white line indicate?

Answer: B — It discourages crossing

A single solid white line serves a purpose in road markings as established by the Manual on Uniform Traffic Control Devices (MUTCD). This line is used to indicate that changing lanes is discouraged. It typically appears alongside lanes moving in the same direction, signaling to drivers that crossing into the adjacent lane may not be safe or permitted. This line helps maintain orderly traffic flow and enhances safety by reducing the likelihood of collisions that can occur when drivers switch lanes inappropriately. Unlike broken lines, which allow for lane changes at appropriate times, the solid white line is a clear directive for drivers to stay in their current lane. Therefore, the selection that states it discourages crossing reflects the protective intent behind this type of road marking.