Choice "A" is correct. Public key infrastructure represents the mechanisms used to issue keys and digital certificates.
Choice "b" is incorrect. Digital certificates are available through commercial certificate authorities, not public key infrastructure.
Choice "c" is incorrect. CA's (Certificate Authorities) include a "tree of trust" to verify identities by checking certificates, not public key infrastructure.
Choice "d" is incorrect. Digital signatures facilitate the creation of legally binding electronic documents, not public key infrastructure.
