Final answer:
The setup item that aids in email deliverability is SPF/DKIM, which are email authentication protocols that verify if the email is sent from an authorized server.
Step-by-step explanation:
The setup item that helps with email deliverability is C. SPF/DKIM. These are email authentication methods that help prevent spoofing and phishing by verifying that the email message is sent from an authorized mail server. SPF (Sender Policy Framework) is a protocol used to declare which IP addresses are allowed to send email on behalf of your domain. DKIM (DomainKeys Identified Mail), on the other hand, provides an encryption key and digital signature that verifies that an email message was not forged or altered.
Setting up SPF and DKIM correctly for your domain is crucial to ensure that your emails are less likely to be marked as spam by receiving email servers, which improves overall email deliverability.