What constitutes an "open" Roth IRA? How should I respond to this question? I corrected, and stopped contributions in the spring, but left eligible funds.

"Open" means that you had a nonzero balance in any Roth IRAs on December 31, 2017 and you did not have any outstanding rollovers of distributions from a Roth IRA or recharacterizations that were not completed until the following year (2018).

Contributions out of your paycheck are not considered a IRA.

So if you did not open an actual Roth IRA and put money in for 2017, then you would answer "No" to this question.