In an isosceles triangle ABC, the triangle ABC = 100 degrees. The degree measure of the angle BAC is …

Let’s consider two possible cases.

1) Suppose the angle ABC is the angle at the base of this isosceles triangle.

Then the second angle at the base must also be 100 °, which cannot be, since in this case the sum of the angles of this triangle will be greater than 180 °.

2) Suppose that the angle ABC is the angle at the apex of this isosceles triangle.

Then the other two angles of this triangle will be equal to each other.

Since the sum of the angles of the triangle is 180 °, the sum of the two angles at the base should be 180 – 100 = 80 °, and the value of one angle at the base, one of which is the angle BAC, is 80/2 = 40 °.

Answer: the correct option is 40 °.