The correct answers are Index storage footprint will grow, Search response latency will increase, and Azure Key Vault must be used.

Enabling server side encryption with customer managed keys causes additional encryption metadata and wrapped key information to be stored alongside index data. This extra metadata increases the amount of stored data which is why Index storage footprint will grow.

Using customer managed keys requires fetching or unwrapping keys from the key store during cryptographic operations which adds processing and network hops. Those extra operations tend to increase response times which is why Search response latency will increase.

Azure Cognitive Search uses Azure Key Vault as the supported place to store and manage customer managed keys so you must use that service when enabling CMK. That is the reason Azure Key Vault must be used.

A self signed X509 certificate must be provided is incorrect because Key Vault holds the key material and access is handled by Key Vault permissions and managed identities rather than by uploading a self signed certificate for the search service.

Index size will shrink is incorrect because encryption adds overhead and metadata so it will not reduce the stored index size.

Query throughput will improve is incorrect because the added cryptographic work and key access usually add overhead and they will not increase throughput.

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