I was active duty Army until 27Dec2016, I joined the National Guard immediately following that. What should I put as my military status as it will only let me choose one

Assuming you are now living in the same state as your Guard unit......(and that you never changed your state-of-legal-residence[SLR] with a DD2058 during your service)

1)  If you were actually in the Guard during 2016 and are actually getting a 2016 guard W-2 for a different state from your Active Duty HOR state, Choose National Guard...then check the box that you left the military during 2016 (that checkbox is for the Active Duty part of the year).

2)   BUT ....but if you aren't getting paid by the NG for 2016, then just choose the Active Duty, and since it was a 27 Dec release, then I'm a bit wishy-washy on the checkbox entry.  ....a)  I'm thinking maybe check the all year box for 2016...and you just file as a resident of your HOR state...b)  then for your 2017 taxes , you will file as a full-yr resident of whatever state you are living in now.

Others may have another opinion on that last part.  IF you left service and returned to your HOR state to joint the Guard..that works perfectly.  IF, however you left service and went to another state to join the Guard (or remained where you separated..but that is a different state from your AD HOR), then technically,  you are supposed to file as a part-year resident of the new state for the final 5 days of the year, and any income during those final 5 days are subject to taxes in the new state (but not the military W-2...that was earned before the 27th ).  Check your DD-214 also...if some terminal leave is associated with your separation, then the date of your separation my be closer to the end of 2016, or even in 2017...then you were in the military all year and your HOR state rules for the entirety of 2016..