Queryable Encryption is designed for client-side protection of sensitive fields while preserving supported query capabilities over encrypted values. With this model, appropriately configured client drivers encrypt selected fields before data is transmitted to the MongoDB server and decrypt authorized results on the client side. That architecture is well suited to highly sensitive PII such as government identification numbers, dates of birth, or regulated demographic information because the database server stores encrypted field values rather than plaintext for those protected fields. The Encrypted Storage Engine, option A, protects database files at rest but the server can still process decrypted data internally. TLS/SSL, option B, protects data while it is moving across the network, not the application field values from the database server itself. Role-Based Access Control, option C, restricts which authenticated principals can perform actions but does not itself cryptographically protect the field contents. Queryable Encryption complements, rather than replaces, TLS, authentication, authorization, auditing, and storage encryption. For the specific requirement of client-side encryption of sensitive application data, option D is the correct MongoDB feature.

Study Guide reference/topic:Security, Networking, and Encryption - Queryable Encryption, client-side encryption, sensitive fields, and key management.

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