Answer
Line segment $\overline{MN}$
Work Step by Step
Draw a point labeled M towards the left of line l. Use your compass to measure the length of segment AB. Align one end of the compass with point M and with the other end make a mark along line l. Now use the compass to measure segment CD. Align one end of the compass with the previously made mark on line l and draw a point, labeled N, where the other end of the compass falls on line l.