A smart card provides an additional factor of authentication by storing secure credentials, such as certificates, that cannot be guessed or replicated. It complements user accounts and is "something you have".
Why Not A (Fingerprint reader): This is "something you are", but the scenario specifically requires a physical token.
Why Not C (Secure token): Secure tokens are also valid but may not integrate as seamlessly into multi-factor authentication for physical and system access.
Why Not D (NFC scanner): An NFC scanner is a device and not a token itself; it reads cards or other credentials.
CompTIA A+ Exam Reference: Core 2 (220-1102), Section 2.5, access controls.