How are fingerprints formed at a crime scene?

Fingerprints are formed at a crime scene primarily through the natural oils, sweat, and other secretions from the skin that reside on the friction ridges of a person's fingertips. When an individual touches a surface, these substances are transferred, leaving behind a unique pattern that can be recognized as a fingerprint.

The friction ridges on fingers are designed by nature to enhance grip and are not smooth; they create the distinctive patterns we observe. The composition of the skin, including both sebaceous secretion (oil) and eccrine perspiration (sweat), allows for a transfer of this material onto various surfaces, where the fingerprint pattern can then be visualized using different imaging techniques or powders.

While other options mention different methods or substances, they do not accurately reflect the biological process of fingerprint formation. Ink, chemical reactions, and blood all suggest unnatural approaches to fingerprint deposition, which do not represent how fingerprints are typically found at crime scenes. Therefore, the correct depiction of the formation of fingerprints through natural skin secretions is the reason why "by friction ridges depositing grease and perspiration" is the accurate choice.