All triangles are polygons. Some polygons are regular. Which conclusion is valid?
Correct: B
The middle term 'polygons' is distributed in the first premise (All T are P) but undistributed in the second premise (Some P are R). This is the fallacy of the undistributed middle. We cannot conclude anything definite about triangles and regularity. The regular polygons may or may not include triangles. The syllogism is invalid.