What component allows a transit to be lined up over a reference point?

The correct component that allows a transit to be lined up over a reference point is the vertical alignment positioner. This device is essential for ensuring that the transit instrument is properly aligned in the vertical plane when measuring angles or elevations. By adjusting the vertical alignment positioner, a user can ensure that the transit is accurately positioned directly above a specified point, which is crucial for precise measurements in construction and surveying.

In the context of the other options, the horizontal alignment positioner is used for ensuring horizontal alignment but does not specifically address vertical alignment over a point. The transit tripod provides stability and support for the transit but does not facilitate alignment over a reference point. Similarly, the leveling plate functions to level the instrument but does not directly contribute to aligning the transit over a specific location.