The reason might me "union involvement".
Step-by-step explanation:
In common a union is formed by organization, to address the employees problems. The union leader will be the bridge between the organization and team of employees.
Option A is invalid because the human resource department need the skill of maintaining relationships. So the skill is matched and the new team can perform.
Lack of human resource expertise: This is the reason why the organization has formed HR department. So the reason for resignation of many employees are not against the set up, but the promotion given to top employees.
Coordinate functions: This is once again not the reason.
Union involvement: Since because two among them got promoted / transferred to a new department, the other employees cannot digest and show there aversion through Union.,