The correct answer is Multi-factor authentication (MFA), which requires users to provide two or more verification methods – something they know (like a password), something they have (like a phone or hardware token), or something they are (like a fingerprint). MFA adds a critical second layer of security to user sign-ins and transactions. Single sign-on (SSO) provides a seamless user experience by enabling users to remember only one ID and password to access multiple applications, but it does not by itself ensure that multiple verification methods are used. Passwordless authentication allows users to sign in without a password by using other methods like a PIN or biometric verification, which supports the concept of MFA, but just offering passwordless options does not imply MFA is enabled. Business-to-business (B2B) and business-to-customer (B2C) refer to identity and access management solutions tailored for managing external user access, not specifically for enforcing multiple verification methods.