What would trigger additional ingestion costs for a customer with 3 APM hosts?

The scenario describes the conditions under which additional ingestion costs would be triggered for a customer using Datadog's Application Performance Monitoring (APM) with three hosts. The correct choice relates to the total amount of data ingested in a month.

Ingesting over 300 GB in total per month surpasses the typical usage limits that are set by Datadog for APM. When usage exceeds these limits, customers are subject to higher costs due to the extra data that goes beyond the included capacity in their plan. This pricing structure encourages efficient use of data and careful consideration of what is being sent to Datadog.

The other options relate to amounts that do not exceed the thresholds that would trigger additional costs. Specifically, ingesting less than 150 GB or exactly 150 GB would not lead to any extra ingestion charges, as they fall within the accepted limits. The notion that only indexing spans trigger costs is also incorrect in the context provided, as ingestion costs relate broadly to the total amount of data processed, not just specific types of data like spans. Thus, the understanding of total ingestion limits is crucial for managing costs effectively.