The correct answer is C. SPF.

SPF stands for Sender Policy Framework, which is a protocol that allows domain owners to specify which IP addresses are authorized to send email on behalf of their domains1. SPF helps prevent email spoofing and phishing by verifying the sender’s identity and preventing unauthorized use of their domain name12.

When an email is received, the receiving server checks the SPF record of the sender’s domain, which is a DNS TXT record that lists the valid IP addresses for that domain12. The server then compares the sender’s IP address with the SPF record and determines if it matches or not. If it matches, the email passes the SPF check and is accepted; if it does not match, the email fails the SPF check and is rejected or marked as spam12.

DKIM stands for DomainKeys Identified Mail, which is a protocol that allows domain owners to digitally sign their emails using a public-private key pair3. DKIM helps ensure the integrity and authenticity of email messages by verifying that they have not been tampered with or forged during transit3 .

MX stands for Mail Exchange, which is a type of DNS record that specifies the mail servers that are responsible for receiving email for a domain. MX records help direct incoming email to the correct destination and prioritize them based on preference values .

DMARC stands for Domain-based Message Authentication, Reporting and Conformance, which is a protocol that builds on SPF and DKIM and allows domain owners to define how receivers should handle unauthenticated or suspicious emails from their domains. DMARC helps protect domains from email fraud and abuse by providing feedback and reports on email delivery and authentication status .

Therefore, SPF is the only service among the options that checks whether the sender’s IP address is associated with the sender’s domain name when receiving email.