Answer: D
Correct answer: It requires a second, independent proof of identity besides the password
Two-factor authentication requires a second proof of identity in addition to the password, so that a stolen password alone is not enough to gain access. The factors are conventionally grouped as something you know - a password or PIN; something you have - a phone, a hardware key, a one-time code; and something you are - a fingerprint, a face. Genuine two-factor authentication uses two different groups; two passwords are not two factors. It is the single most effective defence against phishing and credential theft, because an attacker who obtains a password by any means still cannot log in. The methods differ in strength. SMS codes are the weakest, being vulnerable to SIM-swapping; an authenticator application generating time-based codes is better; and a hardware security key is the strongest, since it verifies the site’s identity too and so cannot be phished at all.