Which of the following is NOT a valid stage in an aggregation pipeline?

In MongoDB’s aggregation framework, each stage in a pipeline performs a specific operation on the documents passing through it. The stages available include operations like $project, which reshapes the documents, $lookup, which allows for joining data from different collections, and $group, which groups documents together based on a specified identifier and applies aggregation functions.

However, the stage $filter is not used as a standalone stage in the aggregation pipeline. Instead, it is a specific operator that can be used within moments like $project or $match to filter elements of an array field. Thus, it does not stand alone as a valid stage in the overall aggregation pipeline architecture.

Understanding these distinctions is crucial for effectively utilizing the MongoDB aggregation framework since employing the correct stages determines how documents are transformed and returned as a result of a query.