Which solution meets this requirement with the LEAST delay?

Explanations:

While creating a webhook to call the CodePipeline API could start the pipeline automatically, it would involve custom implementation and additional management overhead. This is not the most efficient method for automation.

Creating an Amazon EventBridge rule to detect changes in the CodeCommit repository is a straightforward and efficient solution. It allows the pipeline to start automatically with minimal delay upon detecting code changes.

Configuring the pipeline to poll the CodeCommit repository for changes is not optimal as polling introduces delays and might not be instantaneous. Event-driven solutions like EventBridge are preferable for real-time responses.

Setting up a CodeCommit trigger to send messages to an SNS topic to invoke the CodePipeline API adds unnecessary complexity. It requires additional components (SNS) and does not provide the immediacy of EventBridge rules.