Given that Lucia constructed line segment \( \overline{CD} \) as a copy of segment \( \overline{AB} \) using a straightedge and compass, we can conclude that the two segments are equal in length.
Therefore, the correct response is:
\[ AB = CD \]
or
\[ \overline{AB} = \overline{CD} \]
This means that:
\[ \overline{A\ B} = \overline{C\ D} \]