Answer is B and D

B. Ensure that the Organization-Wide Default sharing for Invoices is set to Private.

The requirement states that an Account owner should not automatically see every Invoice. Setting Invoice OWD to Private ensures that only the Invoice owner, their managers (through role hierarchy), and explicitly shared users can access Invoice records.

D. Deploy the Relationship between Accounts and the Invoices Object as Lookup.

A Lookup relationship allows the Invoice object to:

Have its own owner
Have its own sharing model
Be secured independently from the Account

This is essential because users should only see invoices that they or their subordinates own, regardless of Account ownership. In contrast, a Master-Detail relationship would inherit ownership and sharing from the Account.

A and B

"only invoices that they or **their** subordinates own". The user should have access to all the invoice detail records of the Parent. A lookup is not required.

It helps to look at it from the point of view of the Master not the detail owner.

B D
A - is incorrect. Having ownership to account in Master detail relation to invoice will give access to all invoice records. Parent record dictates the ownership to child records. So its not A.
C - is incorrect because workflow cannot create records other than tasks.

Answer would be B & D, Since account owner should not see all Invoice only which they or their subordinates own.
Master detail will control by parent