To let one Prometheus server collect metrics from another Prometheus server is the correct option.

Federation configures a Prometheus server to scrape another Prometheus server so a central instance can gather selected time series from remote instances. It works by targeting the other server's /federate endpoint and by applying label filters to pull only the metrics you need.

This approach creates a hierarchical or global view across clusters without merging underlying storage. Each Prometheus still retains its own TSDB and federation simply makes metrics available to be scraped by another server for cross cluster queries and alerting.

Cloud Monitoring is incorrect because that option names a managed monitoring service and not the purpose of Prometheus federation. Federation is an internal Prometheus scraping mechanism and not a reference to cloud vendor monitoring products.

To combine separate Prometheus time series databases into a single storage is incorrect because federation does not merge TSDBs. Local storage remains independent and you would use remote write to send data to a common long term store if you want unified storage.

To run a high availability pair of Prometheus servers is incorrect because high availability is accomplished by running replicas that scrape the same targets and by designing alerting and external storage appropriately rather than by using federation as a replication mechanism.

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