Without measuring, which triangle has all sides equal to each other

Explanation

The triangle figures in the options show how triangles can be classified according to the length of their sides.  The single tick marks drawn on each side of the equilateral triangle indicate that the sides are of equal length.  The double tick marks drawn on two of the sides o the isosceles triangle indicate that they have the same length.  Each side of the scalene triangle has a different number of tick marks to indicate that the sides have different lengths.