First off, you should know that for any triangle, the 3 angles add up to 180: a + b + c = 180.
Also, if a triangle is isosceles, that means 2 of the 3 angles (as well as 2 of the 3 sides) are the same.
If one of the angles is greater than 100 degrees, neither of the other two can also be greater than 100 (because then just those two angles would add up to more than 200, way more than 180). The other two angles (x) are equal to each other and are way
smaller than 100.
If the angles add up to 180 and one of them is more than 100, the other two must be less than 80 total (again, because they all add to 180). Since the twin angles are the same and add to less than 80, each must be less than 40.
Which means it's d). Hope this helps!