Final answer:
The nurse should instruct the client that her calorie needs should increase by approximately 300 calories a day.
Step-by-step explanation:
A lactating mother needs to increase her calorie intake to support breastfeeding. According to the information provided, a healthy baby may require 850 ml of milk daily, which provides 578 Kcal/day. A lactating mother would need an additional diet of approximately 720 Kcal/day to meet the demands of lactation. Therefore, the nurse should instruct the client that her calorie needs should increase by approximately 300 calories a day (option B).