The plumb is a line with a weight attached to find:

A plumb line is a simple tool used in construction and surveying, consisting of a weight suspended from a string. When the weight is allowed to hang freely, it aligns itself with the direction of gravity, thereby establishing a vertical reference. This tool is crucial in ensuring that structures are built straight and true, particularly when measuring vertical lines or ensuring that elements such as walls and posts are upright.

The reason the correct answer focuses on being a true vertical line is based on the fundamental principle of gravity acting on the weight. The weight ensures that the line is not influenced by other forces, providing an accurate indication of vertical alignment. This precision is especially important in various applications within electrical work and construction, where accurate vertical measurements are vital for safety and structural integrity.