Which figure of speech is used in this text?
"This box weighs a ton!" Martin panted. "Carly, would you mind helping me carry it up the stairs?"

 - understatement
 - hyperbole
The text uses hyperbole, an obvious exaggeration that is not meant to be taken literally.
A ton is an exaggeration, since it would be impossible for one or two people to carry a box that weighed a ton, or 2000 pounds.