
Explanation:

Box 1: No
Each physical partition should have its own index, but since no index is used, the query is not cross-partition.
Box 2: No
Index utilization is 0% and Index Look up time is also zero.
Box 3: Yes
A partition key index will be created, and the query will perform across the partitions.
Reference: https://docs.microsoft.com/en-us/azure/cosmos-db/sql/how-to-query-container