On installation drawings, which dotted line indicates a part in motion or an alternative position?

The correct answer is the phantom line. In technical and installation drawings, the phantom line is used to represent parts that are not in their normal position or to indicate alternate positions or movement of a component. This type of line appears as a series of long dashes and short dashes, which visually distinguishes it from other lines in the drawing.

Hidden lines, on the other hand, are used to indicate edges and outlines that are not visible from the current viewing perspective. Break lines serve to indicate that a part is broken out to show more detail or to save space in the drawing. Dimension lines provide measurements and are typically associated with lines that indicate size rather than motion or alternate positions. Therefore, the phantom line is the appropriate choice for representing objects in motion or alternative positions within installation drawings.