With PGP, which of the following entity signs a users’s public key?

A.
The sender’s administrator who provides the sender with the PGP program

B.
The vendor of the PGP program

C.
The sender of the message

D.
The receipient of the message

E.
A third party that belongs to what’s often known as “web of trust”, that can verify the relationship between the user and the key

Explanation:
Pretty Good Privacy is a computer program that provides cryptographic privacy and authentication. It was originally created by Philip Zimmermann in 1991. PGP and other similar products follow the OpenPGP standard (RFC 4880) for encrypting and decrypting data.
PGP encryption uses public-key cryptography and includes a system which binds the public keys to a user name. The first version of this system was generally known as a web of trust to contrast with the X.509 system which uses a hierarchical approach based on certificate authority and which was added to PGP implementations later. Current versions of PGP encryption include both alternatives through an automated key management server.
Web of trust
Both when encrypting messages and when verifying signatures, it is critical that the public key one uses to send messages to someone or some entity actually does ‘belong’ to the intended recipient. Simply downloading a public key from somewhere is not overwhelming assurance of that association; deliberate (or accidental) spoofing is possible. PGP has, from its first versions, always included provisions for distributing a user’s public keys in an ‘identity certificate’ which is so constructed cryptographically that any tampering (or accidental garble) is readily detectable. But merely making a certificate effectively impossible to modify undetectably is also insufficient. It can prevent corruption only after the certificate has been created, not before. Users must also ensure by some means that the public key in a certificate actually does belong to the person/entity claiming it. From its first release, PGP products have included an internal certificate ‘vetting scheme’ to assist with this; a trust model which has been called a web of trust. A given public key (or more specifically, information binding a user name to a key) may be digitally signed by a third party user to attest to the association between someone (actually a user name) and the key. There are several levels of confidence which can be included in such signatures. Although many programs read and write this information, few (if any) include this level of certification when calculating whether to trust a key.
The web of trust protocol was first described by Zimmermann in the manual for PGP version 2.0:
As time goes on, you will accumulate keys from other people that you may want to designate as trusted introducers. Everyone else will each choose their own trusted introducers. And everyone will gradually accumulate and distribute with their key a collection of certifying signatures from other people, with the expectation that anyone receiving it will trust at least one or two of the signatures. This will cause the emergence of a decentralized fault-tolerant web of confidence for all public keys.
The web of trust mechanism has advantages over a centrally managed PKI scheme such as that used by S/MIME, but has not been universally used. Users have been willing to accept certificates and check their validity manually, or to simply accept them. The underlying problem has found no satisfactory solution.