Given:
Sides of triangles in the options.
To find:
Which could NOT be the lengths of the sides of a triangle.
Solution:
Condition for triangle:
Sum of two smaller sides of a triangle must be greater than the longest side.
In option A,
Sides 5 in, 5 in, 5 in are the lengths of the sides of a triangle.
In option B,
Sides 10 cm, 15 cm, 20 cm are the lengths of the sides of a triangle.
In option C,
Sides 3 in, 4 in, 5 in are the lengths of the sides of a triangle.
In option D,
Since, the sum of two smaller sides is less than the longest side, therefore the sides 8 ft, 15 ft, 5 ft are not the lengths of the sides of a triangle.
Therefore, the correct option is D.