An employee failed to implement encryption to an email before sending a spreadsheet containing credit card numbers to a colleague within the same organization. What PCI DSS requirement has been violated?

Requirement $1$: Install and maintain a firewall configuration to protect cardholder data.

Requirement $6$: Develop and maintain secure systems and applications.

Requirement $4$: Encrypt transmission of cardholder data sent via end$-$user messaging technologies

Requirement $8$: Restrict access to cardholder data by business need$-$to$-$know.