Final answer:
When tax returns are filed electronically, a taxpayer can sign the return using a digital signature.
Step-by-step explanation:
When tax returns are filed electronically, one way a taxpayer can sign the return is through a digital signature. A digital signature is a mathematical scheme that verifies the authenticity and integrity of a message or document. It provides a secure way of confirming the identity of the person signing the return and ensures that the document has not been tampered with.