How can a writer indicate that others have received a copy of a letter?

The indicated choice is correct because adding a lowercase "c" followed by names is a traditional way to inform the primary recipient that others have received a copy of the letter. This practice is known as "carbon copy," abbreviated as "cc." When a writer includes "cc" at the bottom of a letter or email, it clearly communicates to the main recipient who else is receiving the correspondence, ensuring transparency.

This method is widely recognized and commonly used in business communications, allowing all parties to be aware of who else is informed about the discussion or information being conveyed. While the other options may facilitate communication in other ways, they do not serve the same purpose of indicating shared receipt of the letter as effectively as using "cc."