Final answer:
A Raman-active vibrational mode requires a change in the polarizability of a molecule during vibration.
Step-by-step explanation:
The specific property that must change during a molecular vibration for that vibrational mode to be Raman-active is the polarizability of the molecule. The change in polarizability essentially means that there must be a change in how the electron cloud around a molecule's bond is distorted when exposed to incident light. Unlike infrared activity, which requires a change in the dipole moment, Raman activity depends on the change in polarizability. This is why certain molecular vibrations are only visible in Raman spectroscopy and not in infrared spectroscopy.