The correct answer is B. It checks the sending IP address is authorized by the sender's domain. Proofpoint's SPF reference states that an SPF record in DNS specifies which IP addresses and hostnames are authorized to send emails for a domain. When the receiving mail server evaluates SPF, it checks whether the source server is on that authorized list. If it is not, the message can fail SPF and be treated as suspicious, spam, or rejected according to policy.

Proofpoint's broader email-authentication overview describes the SPF step in almost the same way: the receiving server verifies that the sending IP address is approved to send emails for the domain. That is the exact function being tested in this question. SPF is not about validating the recipient, and it is not the mechanism that checks a cryptographic message signature. Those are different controls. DKIM is the mechanism associated with digital signatures over message content and headers, while ARC deals with preserving authentication assessments across forwarding paths.

Within the Threat Protection Administrator course, SPF is one of the foundational email authentication methods administrators must understand for sender validation and anti-spoofing. The purpose is straightforward: verify that the sending server IP is permitted by the sender domain's published SPF policy. Therefore, the correct course answer is B.