To create a read-only role for certain employees working in the human resources department, the role needs to have the following permissions on the hr_db database:

USAGE on the database: This allows the role to access the database and see its schemas and objects.

USAGE on all schemas in the database: This allows the role to access the schemas and see their objects.

SELECT on all tables in the database: This allows the role to query the data in the tables.

Option A is the correct answer because it grants the minimum permissions required for a read-only role on the hr_db database.

Option B is incorrect because SELECT on schemas is not a valid permission. Schemas only support USAGE and CREATE permissions.

Option C is incorrect because MODIFY on the database is not a valid permission. Databases only support USAGE, CREATE, MONITOR, and OWNERSHIP permissions. Moreover, USAGE on tables is not sufficient for querying the data. Tables support SELECT, INSERT, UPDATE, DELETE, TRUNCATE, REFERENCES, and OWNERSHIP permissions.

Option D is incorrect because REFERENCES on tables is not relevant for querying the data. REFERENCES permission allows the role to create foreign key constraints on the tables.

https://docs.snowflake.com/en/user-guide/security-access-control-privileges.html#database-privileges

https://docs.snowflake.com/en/user-guide/security-access-control-privileges.html#schema-privileges

https://docs.snowflake.com/en/user-guide/security-access-control-privileges.html#table-privileges