Isosceles triangles is have two congruent side and one different side. Also, they have two congruent angles and the otherangle is different.
Then you have to choose the option with two congruent sides.
If you have an option with three congruent sides, do not choose it becuase that is an equilateral triangle.
If you have these options:
A) 3,4,5
B) 12,12,12
C) 16,16,18
D) 19, 20, 20.5
Only the option C) may be of an isosceles triangle.
Option B is an equilateral triangle.
