Which of the following stages is specifically designed to filter documents in an aggregation pipeline?

The stage specifically designed to filter documents in an aggregation pipeline is the $match stage. This stage is used to specify criteria that documents must meet to be included in the output of the aggregation. It operates similarly to the find query in MongoDB, allowing you to apply conditions such as equality, range queries, and logical operators to filter the documents in the input stage of the pipeline. By using $match, you can significantly reduce the amount of data that flows through the subsequent stages of the pipeline, improving performance and efficiency.

On the other hand, $set is used to add new fields or modify existing fields in the documents. $project is utilized for reshaping the documents by specifying which fields to include or exclude, and potentially creating new computed fields. $out allows you to write the results of the aggregation pipeline to a specified collection but does not perform any document filtering itself. Thus, $match is the correct choice for the purpose of filtering documents in an aggregation pipeline.