Final answer:
According to the NIST scale, the incident should be classified as Medium.
Step-by-step explanation:
According to the NIST scale, Ben should classify the functional impact of the incident as Medium (C).
The NIST scale categorizes the functional impact into four levels: None, Low, Medium, and High. In this case, since approximately 30 percent of the organization's staff is unable to access email service, it can be considered a significant disruption to their work. However, it does not completely disable the organization's ability to function, so it falls under the Medium category.
By classifying the incident as Medium, Ben can prioritize the necessary actions and resources to address the issue and restore email service for the affected staff.