Geometry

Question Analysis:
This question focuses on classifying triangles based on angle measures, specifically on identifying acute - angled triangles.
The main goal is to test the students' understanding of the acute - triangle definition.

Key Concept Explanation:
An acute - angled triangle has all angles less than 90°.

Step - by - Step Solution:
Analyze option A: All angles 40°, 60°, and 80° are less than 90°, so it is an acute - angled triangle.
Analyze option B: It has an angle of 90°, so it is a right - angled triangle, not an acute - angled triangle.
Analyze option C: It has an angle of 100° which is greater than 90°, so it is an obtuse - angled triangle, not an acute - angled triangle.
Analyze option D: All angles 55°, 65°, and 60° are less than 90°, making it an acute - angled triangle.

Common Mistakes:
Students may misinterpret the angle measures or be con...