Answer:D.
maintain a concerned expression
Step-by-step explanation:
One of the main traits of a successful Custer service agent is empathy. When the customer is angry and one isaintaining a happy or pleaseant face, he will interprete it as lack of concern for his plight.
The best expression is to maintain a concerned expression that shows you sympathise with his situation and you have personalised the situation.
This sympathy is expressed as doing your best to effectively solve the customer problem.