While there are a limited number of operating systems you need to be familiar with, this question is best answered after researching the organization's technology infrastructure. Discussing operating systems that the company doesn't use can be a waste of time. Your answer should focus on the operating systems they use, your familiarity with them, and your experience supporting them in your past positions. The information you need can come from the company's web site, conversations with the vendors they use, or informational interviews with past and present employees.