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.