What component of an aggregation pipeline do documents pass through for processing in sequence?

The correct answer highlights that documents in an aggregation pipeline flow through aggregation stages for sequential processing. In MongoDB, an aggregation pipeline consists of a series of stages, each transforming the documents as they pass through. These stages, which can include operations like filtering, sorting, grouping, and projecting fields, allow users to perform complex data manipulations and computations on the data.

Each stage takes the output of the previous stage, processes it, and then forwards the result to the next stage. This sequential processing is fundamental to how the aggregation framework operates, enabling developers to build powerful queries that can summarize, reshape, and analyze large volumes of data efficiently.

The other options do not represent the processing flow of documents within an aggregation pipeline. For instance, collection methods refer to the ways in which collections are accessed and manipulated, while document types denote the structure of documents themselves. Index structures are related to optimizing query performance but do not pertain to the direct processing sequence of documents in an aggregation context. Therefore, only aggregation stages accurately describe the sequential path documents follow during processing in an aggregation pipeline.