Which MongoDB functionality allows setting rules for field existence before document creation?

Data Validation in MongoDB refers to the capability to enforce rules and constraints on the data in collections before documents are created or updated. This functionality allows developers to define specific schemas for their collections, including requirements for field existence, data types, and value constraints. By using JSON Schema, you can specify which fields must be present in a document and what types of values those fields should contain. This feature ensures data integrity and consistency right from the point of data input, preventing invalid or incomplete documents from being stored in the database.

The other options, while important in the context of MongoDB, do not specifically address the requirement for setting rules on field existence before document creation. Schema Design pertains to how models and structures are created for data organization, but it does not enforce validation rules at the database level. Indexing is focused on optimizing query performance and does not concern itself with data integrity. The Aggregation Framework is designed for processing and transforming data within collections, typically after the documents have been created or updated, rather than preventing the Creation of documents that do not meet certain criteria. Thus, Data Validation is the functionality that specifically allows for the establishment of such rules.