The ability of X-rays to penetrate tissues like skin and their comparatively lower ability to penetrate bone is attributed to their short wavelengths.
X-rays have short wavelengths, which allows them to interact with matter in a way that is effective for imaging. Bones, being denser than soft tissues, absorb more X-rays due to their density and atomic composition, resulting in less penetration compared to less dense tissues.
So, the correct response is: their short wavelengths.