Final answer:
The best response the cashier can make is to apologize and explain the store's staffing situation.
Step-by-step explanation:
The BEST response the cashier can make in this situation is B. Apologize and explain the store's staffing situation. By apologizing, the cashier acknowledges Carrie's frustration and shows empathy. Explaining the store's staffing situation can help Carrie understand the reason for the long checkout lines and may diffuse any further arguments or complaints.