The correct answer is C, because:

In Salesforce Marketing Cloud, when a subscriber's status changes to 'Held', it means that Marketing Cloud has suppressed sending emails to that subscriber due to repeated soft or hard bounces.

Users cannot directly edit or re-enable subscribers marked as Held.

Instead, Salesforce Support must be contacted to review and re-enable those subscribers if appropriate (for example, if it was a temporary deliverability issue).

Salesforce official documentation states:

"Held Status: A subscriber is placed on Held status after a threshold of consecutive soft or hard bounces. To reactivate held subscribers, submit a case with Salesforce Support. Users cannot manually reactivate held subscribers themselves."

(Source: Salesforce Marketing Cloud - Subscriber Status Overview)

Important Points:

Held status is a safety mechanism to protect sender reputation and avoid deliverability issues.

Salesforce must verify that it is safe to re-enable these subscribers to prevent risking future blacklistings or IP reputation damage.

Why the other options are incorrect:

❌ A. Extract and Import as 'Active':

You cannot change subscriber status from 'Held' back to 'Active' through data imports — importing won't override Held status.

❌ B. Mass update in Contact Builder:

Contact Builder cannot change the status of subscribers in the All Subscribers list — status is managed at the system level.

❌ D. SQL query to update 'Held' to 'Active':

SQL queries cannot modify system-managed fields like subscriber status in the All Subscribers list.

Additional reference:

"Marketing Cloud Support can reset a subscriber's status to Active after evaluation. Self-service options are not available to update 'Held' statuses."

(Source: Salesforce Marketing Cloud Admin Guide - Managing Subscribers)

Thus, C is the correct and official answer.