JSON isn't a class in C#. It's a standard format for data serialization. You'd either return an object to be serialized by consuming code:
public List<Employee> GetListOfEmployees()
{
// return a List<Employee>
}
Or return a string representing the serialized object:
public string GetListOfEmployees()
{
// serialize a List<Employee> to a string and return the string
}
As for how to serialize the object, there are a variety of ways to do that. You could even return a string literal that you manually wrote which represents the object, it makes no difference to the consuming code as long as it's valid JSON.