Final answer:
The most appropriate tool for sending a Chatter notification after a record update in Salesforce is the Process Builder, since it allows for automating this specific action efficiently.
Step-by-step explanation:
To send a chatter notification when a record is updated in Salesforce, the best automation tool to use would be Process Builder. While Workflow Rules can send emails, Tasks, and Field Updates, they do not support posting to Chatter. Flow is a powerful tool for complex automation, but for the specific task of sending a Chatter notification upon record update, Process Builder is the more straightforward choice. The Approval Process is used for records that require approval and is not typically used for automating Chatter notifications.