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 first and the last set do not represent triangle at all, because the sum of two lengths is equal to the third length:

3 + 6 = 9;

5 + 5 = 10.

The second set represent equilateral triangle, because all sides have the same lenghts. Each equilateral triangle has 3 lines of reflections. These lines are exactly three medians of the triangle (because each median in equilateral triangle is the perpendicular bisector of the base of the triangle and divides the triangle in two congruent right triangles).

correct choice is B - set (5,5,5)

As the bottom answer stated it would be C) (5, 5, 5) Not B.

3 is greater then both of those numbers