What is the reason that a digital signature can be used to provide the authenticity of digitally signed data?

What is the reason that a digital signature can be used to provide the authenticity of digitally signed data?
A . Both the signer and the recipient must first agree on a shared secret key that is only known to both parties.
B . Both the signer and the recipient must first agree on the public/private key pair that is only known to both parties.
C . Only the signer has sole possession of the private key.
D . Only the recipient has a copy of the private key to decrypt the signature

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Answer: C