The stimulus discusses the relative rates of the syndrome, so the total numbers of employees in each department aren't important to the argument.

Even if there can be other causes, are any of those causes likely to arise in Department F? Without that information, this doesn't affect the argument.

This confirms the assumption that Department F isn’t just comprised of vulnerable individuals. This strengthens by supporting the causal connection between Department F and the syndrome.

We don’t care how common the syndrome is among transportation employees, generally. We’re interested in the relative rates between departments, so this isn't relevant.

We need something that differentiates Department F from the rest of the company. This doesn’t do that because it applies to the whole company, so it's irrelevant.