Final answer:
The primary nurse anticipates that the drainage container will be kept level with the affected arm after a mastectomy.
Step-by-step explanation:
The primary nurse would anticipate that the drainage container will be kept level with the affected arm, which is option A. This is important because keeping the container level helps with the proper drainage of fluids after a mastectomy. It prevents any backflow or pooling of fluids in the affected area. By keeping the container level, the nurse ensures appropriate management of post-operative care for the client.