What is the "facility of a question"?
Solution 1:
facility often indicates:
ease of doing or making; absence of difficulty
Collins online
In this case, it is a statistical term relating to test and survey items and their relative ease of being answered correctly. It usually equals the mean.
chart: moodle.org
It is sometimes referred to as "item difficulty". When the index is too low, it might indicate that the test was bad, and should be discarded. On the other hand, this metric can be (ab)used by teachers to adjust overall performance of a group by skewing tests to raise the class average.