Answer: Unfreezing stage
Step-by-step explanation: Under the unfreezing stage the organisation tries to detect the problems in operational services hindering the business, in this the organisation tries to know the current status of the organisation in the market and the needs of the customers which are not getting fulfilled. After realizing the needs and requirements of customers the plans, goals etc of the organization are changed accordingly.
In the given case the company is detecting the problems in customer service so the unfreezing stage would be an appropriate stage for Mary to provide this problem to employees.