The correct option is Range index.

Range index is appropriate because it supports efficient filtering and sorting over intervals for numeric and string values, which is what you need for time based queries on the recorded_time attribute. In Azure Cosmos DB a range index enables range scans and order by operations so queries that filter or sort by time will perform better when that index is present.

Spatial index is incorrect because spatial indexes are intended for geospatial queries such as distance calculations and polygon containment and they do not optimize range scans or ordering for a timestamp field.

Hash index is incorrect because hash indexes are optimized for exact match or equality lookups and they do not support efficient range queries or sorting that are needed for time interval filters.

Unique index is incorrect because unique indexes enforce value uniqueness across documents and they are not designed to improve performance for range filtering or sorting by time. Use a unique index only when you must guarantee that no two documents share the same value for an attribute.

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