The correct option is Assign Azure role based access control permissions to each application for the search resource.

Assigning Azure role based access control permissions lets you grant each client application only the specific actions it needs on the search service. You can use built in roles or create custom roles to limit operations such as querying, indexing, or managing indexes and then assign those roles to the applications. When applications authenticate and present an identity the role assignments on the search resource determine which operations are allowed.

Create a private endpoint for the search service is incorrect because a private endpoint only restricts network access to the service. It does not provide fine grained authorization to control which search operations each application can perform.

Enable Azure Active Directory authentication and use managed identities is incorrect on its own because authentication proves identity but does not by itself limit which operations are permitted. You still need to assign RBAC roles to the identities to enforce per application permissions on the search resource.

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