The original line segment was divided into two equal pieces by the line segment bisector, each measuring 2.9 centimeters. To find the length of the original line segment, we need to add the lengths of the two equal pieces together:
\[ \text{Total Length} = 2.9 , \text{cm} + 2.9 , \text{cm} = 5.8 , \text{cm} \]
Therefore, Andy's original line segment was 5.8 cm long.
The correct response is 5.8 cm.