The third angle of the triangle will be the supplement of the sum of the given angles. In order for the triangle to be isosceles, two of the three angles must have the same value.
a) 180 -40 -75 = 65 . . . not isosceles
b) 180 -30 -100 = 50 . . . not isosceles
c) 180 -35 -70 = 75 . . . not isosceles
d) 180 -50 -80 = 50 . . . matches one of the other angles. This triangle is isosceles.
