Explanation:
Authentication systems or methods are based on one or more of these five factors: Something you know, such as a password or PIN Something you have, such as a smart card, token, or identification device Something you are, such as your fingerprints or retinal pattern (often called biometrics) Something you do, such as an action you must take to complete authentication Somewhere you are (this is based on geolocation)

Writing your signature on a document is ‘something you do’. Someone can then analyze the signature to see if it matches one stored on record.