An ML engineer has a custom container that performs k-fold cross-validation and logs an average...

Amazon SageMaker Automatic Model Tuning extracts objective metrics directly from training logs. For custom containers, AWS requires the ML engineer to define a metric definition that specifies a regular expression to parse the desired metric value from standard output.

The ObjectiveMetricName in the tuning job must match the metric captured by the regex. This is the only supported method for integrating custom metrics with SageMaker AMT.

TrainingInputMode does not define metrics. CloudWatch metrics cannot be used as tuning objectives. AMT cannot read metrics from S3 artifacts.

AWS documentation explicitly states that regex-based metric definitions are required for custom metrics in hyperparameter tuning jobs.

Therefore, Option B is the correct and AWS-verified solution.