Reddit user FrenchGuy83 recommended chose no, as attending the Defence Preparation Day isn't really serving in, helping, or participating in a military unit:
I checked 'No' because you didn't actually enroll or join a military unit that day - there's no military training of any sort. I saw it more like attending a job/career fair and learning more about your options should you decide to join the army.
Reddit user rrrhao recommended the followingalso recommend to be consistent across forms:
Did you ever filed a I-485? If so, match the answer on the I485.
The I-485 form (mirror) does have some similar questions on military involvement indeed:
