C: All isosceles triangles are not similar. The pair of congruent angles within one triangle is not necessarily congruent to the pair of congruent angles within the other triangle.
Step-by-step explanation:
An Isosceles triangle is one that 2 of it's sides are equal and thus two of its angles are congruent which means they are equal.
Now, another Isosceles triangle may not necessarily be similar to another one because the congruent angles in one Isosceles triangle may not be the same with that of another Isosceles triangle.
