Well the sum of three angles of a triangle is 180 degrees. So the other two angles must add upto 80 degrees. Since it is an isosceles triangle, two of its angles must be equal. So the remaining two angles add upto 80 and are equal so each must be 40 degrees. Hence A is sufficient.

B just lists one angle is 40 degrees. Now the other two could be 70 each or one angle could be 40 and the other 100. So B is not sufficient.