Tina described four triangles as shown below:
triangle a: all sides have length 12 cm.
triangle b: two sides have length 10 cm, and the included angle measures 60°. triangle c: base has length 15 cm, and base angles measure 40°.
triangle d: all angles measure 60°.
which triangle is not a unique triangle? triangle a triangle b triangle c triangle d

Triangle D is not a unique triangle.

Step-by-step explanation:

Triangle A:  All sides have length 12 cm.

such a triangle will be a unique triangle and it will be an equilateral triangle.

Triangle B: Two sides have length 10 cm, and the included angle measures 60°.

Such a triangle will also be unique because with the help of the given angle measure and and the angles opposite to equal sides are equal we can find the measure of those two angles.

and hence we can construct a triangle with the help of these information.

Triangle C:  Base has length 15 cm, and base angles measure 40°.

Such a triangle will also be unique as a unique triangle could be constructed with the help of these information.

Triangle D: All angles measure 60°.

Such a triangle will be an equilateral triangle but we can't say it will be unique as we can construct infinite number of triangles with different side lengths.

Hence such a triangle is not unique.