Comprehensive and Detailed In-Depth Explanation:

Authentication methods are categorized into three factors:

Something you know (e.g., passwords, PINs).

Something you have (e.g., ID cards, key fobs, smart cards).

Something you are (e.g., biometrics like fingerprints, retina scans).

Option C (A card-key scanner) aligns with "something you have", as it requires a physical token (card) for authentication.

Option A (Retina scan) and Option D (Fingerprint scanner) fall under biometric authentication ("something you are").

Option B (PIN code reader) is based on "something you know".

Thus, C is the correct answer because a card-key represents a physical access control mechanism based on possession.