Final answer:
Amanda should use her private key to sign the email, as it ensures the authenticity and integrity of the email.
Step-by-step explanation:
Amanda should use her private key to sign the email. A digital signature is created using a cryptographic algorithm that combines Amanda's private key with the contents of the email. This ensures the authenticity and integrity of the email, as only Amanda's private key can generate a valid signature that can be verified using her public key.