The best option for obtaining a sample that can give the most valid generalization about the team-building activity preferences of all employees is:
400 randomly chosen employees from the list of all employees.
This option ensures that every employee has an equal chance of being selected, which increases the likelihood that the sample is representative of the entire company. The other options are biased in some way, such as focusing on a specific gender, a limited number of departments, or tenure within the company, which would not provide a well-rounded view of the preferences of all employees.