A. Swarming is a feature within Salesforce for internal knowledge sharing, not directly related to collaboration on specific cases.

B. A case team workspace in Slack creates a dedicated space for communication, but it doesn't automatically integrate with Salesforce cases.

C. A case notification Slackbot would directly address the need. It can notify relevant service team members within Slack when a case is assigned, updated, or requires attention. This fosters real-time collaboration on cases leveraging their existing communication platform.

Therefore, the best recommendation is:

C. Configure a case notification Slackbot.