This question tests middle school quantitative reasoning skills: classifying shapes by properties (aligned with ISEE standards). Understanding shape classification involves recognizing defining properties such as side lengths, angles, and symmetry. An equilateral triangle is defined by having all three sides equal in length, which also results in all three angles being equal (60 degrees each). The correct choice, 'Equilateral triangle,' is distinguished from other triangle types: right triangles (one 90-degree angle), scalene triangles (no equal sides), and isosceles triangles (exactly two equal sides). Students often confuse isosceles and equilateral triangles, not realizing that 'equilateral' means ALL sides are equal. To improve, students should practice measuring triangle sides and angles, using the prefix 'equi-' (meaning equal) as a memory aid. Drawing and labeling different triangle types helps visualize their unique properties.