Final answer:
The HR manager should hold employee meetings to acknowledge past issues and ask for an opportunity to improve workplace conditions, respecting employees' right to pro-union sentiment.
Step-by-step explanation:
Given the complexities of a corporate acquisition and workplace culture, the HR manager should approach the situation with care and professionalism. Upon discovering a significant degree of pro-union sentiment among the employees of the acquired company, the HR manager should opt for option D: Conduct employee meetings, acknowledge the past shortcomings, and request an opportunity to make improvements. This approach will help build trust, address employee concerns transparently, and demonstrate a commitment to a more positive work environment. It is also important to engage with employees in a way that does not impede their right to discuss or organize a union, as doing so could be in violation of labor laws.