Final answer:
When an employer receives a notification about a trade union's Application for Certification, they should seek legal advice to navigate labor laws appropriately.
Step-by-step explanation:
Once an employer is notified that a trade union has made an Application for Certification, the appropriate course of action is to seek legal advice (option 3). Ignoring the notification, contacting the trade union for more information, or taking no action could all have serious legal implications for the employer. It is critical for employers to understand their legal rights and obligations in such situations. Seeking professional guidance helps in navigating the complexities of labor law and ensuring that the employer's response adheres to legal requirements and union formation procedures, such as those established in the United States and Canada.