Which of the following is a sign of labor onset?

Lightening is a key sign of labor onset as it refers to the process where the fetus drops lower into the pelvis in preparation for birth. This often occurs several weeks before labor starts and is characterized by the mother feeling a relief in pressure on her diaphragm, making it easier to breathe. Additionally, lightening may increase pelvic pressure and the urge to urinate more frequently as the fetus positions itself for delivery.

The other choices do not indicate the onset of labor. Decreased fetal movement might raise concerns and may require further evaluation, increased appetite is not typically associated with labor preparation, and higher energy levels could be a part of the nesting instinct some women experience before labor but is not a reliable sign of labor onset. Thus, lightening stands out as a clear indicator that labor is approaching.