Explanation/Reference:
Identity Store: It is directory server to perform the authentication. It contains user name , password and groups membership information .When a user name and password combination is entered at log in, the authentication provider searches the identity store to verify the credentials provided.
See figure below.

Incorrect answers:
A: Policy Store: Contains the definition of Application Roles, Application Policies, and the mapping between them.A policy store can be file-based or LDAP-based.Oracle Business Intelligence permissions are granted by mapping users and groups from the identity store to Application Roles and permission grants located in the policy store.
B: Credential Store: It is responsible for securely storing and providing access to credentials required by Oracle Business Intelligence Applications components internally.For e.g, SSL Certificates stored here.