Question

in an isosceles triangle that is not equilateral, the angle between the congruent sides is called a _______angle. a.leg b.vertex c.base d.corollary

Answer 1

In an isosceles triangle this is not equilateral, the angle between the congruent sides is called a vertex. 
B.Vertex

Answer 2

we know that

An isosceles triangle is a triangle with two congruent sides. These congruent sides are called legs. The point at which these legs meet is called the vertex point of the isosceles triangle, and the angle formed by the legs is called the vertex angle. The other two angles of the triangle are called base angles.

see the attached figure to better understand the question

therefore

the answer is

Vertex angle