<u><em>Answers:</em></u><span>The base angles are congruent.
</span><span>The two sides opposite the base angles are congruent.
</span>The bisector of the vertex angle is the perpendicular bisector of the base.<span>
<u><em>Explanation:</em></u><u>An isosceles triangle</u> is defined as a triangle which has two equal sides and consequently two equal angles
<u>Let's check the choices:</u></span><span><u>The base angles are congruent:</u> Correct as mentioned above
</span><span><u>All three angles are congruent.:</u> Incorrect as we mentioned that only two angles are equal. An equilateral triangle is the one having three equal angles
</span><span><u>The two sides opposite the base angles are congruent:</u> correct as mentioned above
</span><u>All three sides are congruent:</u> Incorrect as we mentioned that only two sides are equal. An equilateral triangle is the one having three equal sides
<u>The bisector of the vertex angle is the perpendicular bisector of the base:</u> correct as when bisecting the isosceles triangle, two congruent right triangles are formed
Hope this helps :)