Final answer:
An X.509 certificate is part of a Public Key Infrastructure, and its attributes include a unique serial number, issuer data, validity period, subject information, and associated public key.
Step-by-step explanation:
The choice that identifies attributes required for an X.509 certificate is b) Public key infrastructure (PKI). An X.509 certificate is a standard format for public key certificates, which are used in secure communications and rely on a PKI to verify that the public key contained in the certificate belongs to the entity noted in the certificate. Key attributes of an X.509 certificate include, but are not limited to, a unique serial number, issuer signature, algorithm used, issuer's name, validity period, subject's name, subject's public key, and sometimes certain extensions for various purposes.