Which set of side lengths represents a triangle with 3 lines of reflectional symmetry?
3, 4, 5
3, 6, 9
5, 5, 5
5, 10, 5

The correct option is (C) 5, 5, 5.

Step-by-step explanation:  We are given to select the correct set of side lengths that represents a triangle with 3 lines of reflection symmetry.

We know that

A triangle will have three lines of symmetries of reflection if and only if all the three sides are equal in length. That is, the triangle must be an equilateral one.

Out of the given options, only the set (C) 5, 5, 5 represents the side lengths of an equilateral triangle.

Therefore, the correct set of side lengths representing a triangle with 3 lines of reflection symmetry is 5, 5, 5.

An equilateral triangle ABC with three lines of reflection symmetry a, b and c is shown in the attached figure below.

Option (C) is CORRECT.