When rounding to the nearest thousand, you look at the following number to know if you go up or stay. In the case of 9,991:
9,<u>9</u>91 <em>If the 2nd largest digit is ≥ 5, then round up.</em>
10,000
Therefore, you would round 9.991 to 10,000.
<em>Hope this helps! :)</em>
<u><em>Edit: I literally solved for the wrong thing earlier lol; fixed it.</em></u>