Option A says "after the assignment to notify the assigned user". But the question says when gets assigned. So the option B is the correct option.

I think the answer is A not B. Assignment notifications
An assignment notification is an email or push notification that your application sends when work is assigned to a user or the service-level agreement for an assignment has expired. You can use assignment notifications to call attention to unresolved work and help users resolve cases faster.
By default, assignments inherit the notification policy that is defined by the case type. For example, you can configure a case type to send email notifications each time that an assignment is routed to a worklist. You can override this policy at the assignment level by disabling notifications or providing a custom implementation.
Assignment notifications are not sent in the following cases:
 A user pulls an assignment from a work queue.
 A user transfers ownership of an assignment to another user.
 The assignment is routed to the worklist of the user who is currently processing the case.