Which of the following correctly lists the types of triangles based on their sides and angles?

• A. Acute, obtuse, right, left, top, bottom, equilateral, isosceles, scalene, circular, square, rectangular
• B. Only equilateral, isosceles, and scalene triangles exist
• C. $\text{Equilateral, isosceles, scalene (by sides); acute, right, obtuse (by angles)}$
• D. Equilateral, isosceles, obtuse, right, scalene, acute, congruent, similar, parallel, perpendicular, regular, irregular

Answer: (C)

Verified step by step guidance

1. Understand that triangles can be classified based on their sides or their angles.
2. Classification by sides includes: equilateral (all sides equal), isosceles (two sides equal), and scalene (all sides different).
3. Classification by angles includes: acute (all angles less than 90°), right (one angle exactly 90°), and obtuse (one angle greater than 90°).
4. Recognize that terms like 'left', 'top', 'bottom', 'circular', 'square', 'rectangular', 'congruent', 'similar', 'parallel', 'perpendicular', 'regular', and 'irregular' are not standard classifications of triangles by sides or angles.
5. Therefore, the correct classification combines the side-based types (equilateral, isosceles, scalene) with the angle-based types (acute, right, obtuse).