An employee tries to mask his identity by programmatically changing the From field to $"$a$-$nony$-$mouse" in an email. He uses periodic intervals to send the email to the entire company promoting a product he is selling online. An investigation reveals who sent the message. What capability of cryptography was used to confirm the sender$\text{'}$s identity?

a$.$ Metadata

b$.$ Obfuscation

c$.$ Repudiation

d$.$ Nonrepudiation