What sort of security does a Public Key Infrastructure (PKI) offer?

A.
It provides digital certificates which can be used to digitally sign documents. Such signatures irrefutably determine from whom a document was sent.

B.
Having a PKI shows customers that a web-based business is secure.

C.
By providing agreements, procedures and an organization structure, a PKI defines which person or which system belongs to which specific public key.

D.
A PKI ensures that backups of company data are made on a regular basis.

Explanation:
PKI is based on public key cryptography, and includes much more than just the cryptography. A characteristic of a PKI is that through agreements, procedures and an organization structure, it provides guarantees regarding which person or system belongs to a specific public key.