Lets go over each triangle to find if it is unique or not,
Triangle A
All sides have length 12 cm
Under side-side-side property, as we know the length of all three sides of the triangle
Therefore it is an unique triangle
Triangle B
Two sides have length 10 cm and the included angle measures 60 degrees
This falls under the Side-Angle-Side property, where we know the length of 2 sides of a triangle and an included angle of the triangle
Therefore it is an unique triangle
Triangle C
Base has length 15 cm, and base angle measures 40 degrees
This goes under the Angle-Side-Angle property, where we know measurements of two angles and an included side
Therefore it is an unique triangle
Triangle D
All angles measure 60 degrees
For this, we know the measurements of all the angles but we don't know the measurements of any sides.
Therefore it is a non-unique triangle
The answer to this question Triangle D
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