As mentioned in question her grade in first five tests between 90 and 100 ie. 91,92,93,94,95,96,97,98,99.
Then if she get average 99 grade on first five tests then,
(99+99+99+99+99+x)/6 = 89
x = 39 , where x is grade in sixth test.
and if she get average 91 grade in first five test then,
(91+91+91+91+91+x)/6 = 89
x = 79 , x= grade in sixth test
Therefore, as her grade in first five test were between 90 and 100 (ie. 91......99), then the grade at sixth test may vary from 39 - 79
So, the answer from option 4 to 8
Option 4. ---35 option 4 seems as it is from 36 to 45
Option 8. ---85 option 8 seems as it is from 76 to 85
The question is so confusing as it did not give any clear option.
As the actual answer is 39 to 79