A digital signature is a mathematical scheme for demonstrating the authenticity of a digital message or document. A valid digital signature gives a recipient reason to believe that the message was created by a known sender and that the sender cannot deny having sent the message (non-repudiation). It is the correct answer because it binds each signature to the document with a unique fingerprint (hash), which, if altered, invalidates the signature. A handwritten signature on a digital contract is not verifiable and is unsuitable for digital transactions. Using a company seal doesn't ensure non-repudiation, as it can't uniquely link the document to its sender. Simple acknowledgment by email is not secure and does not provide non-repudiation, because email messages can be easily spoofed.