Explanation:

Quick Actions are the recommended method for launching a Slack Swarm from a case in Salesforce. A Quick Action can be configured on the Case object to initiate a flow or a custom component that integrates with Slack. This allows an agent to easily and consistently start a swarm with a single click directly from the case record, bringing in the necessary experts and providing them with context from the case.

Escalation rules (A) are used to automatically escalate a case when it meets specific criteria (e.g., time elapsed, no response). They are not designed for initiating a collaborative effort like a Slack swarm.

Apex trigger (B) is custom code that executes before or after a record is inserted, updated, or deleted. While it's possible to use an Apex trigger to interact with Slack, it's a more complex and less user-friendly approach for an agent-initiated action. A Quick Action provides a declarative, easy-to-use interface for the agent.

Reference:
For more information on integrating Salesforce with Slack and using actions, you can refer to the Salesforce documentation on Salesforce for Slack and Quick Actions. The Slack Integration documentation specifically discusses how to use Salesforce flows and actions to enable collaborative features like swarming.