No.4- This is not a triangle since one side should be less than the sum of the 2 other sides. In this case 8=3+5, hence it's equal & not smaller
N0.5- This is an isosceles triangle & there are an infinity of triangles with the mentioned degrees. (these are called similar triangle)
No6- This is a unique triangle
No.7- If the plane is parallel to the bases, then the slicing is a cercle.
But if the plane (Parallelogram) is tilt, then the slicing is an ellipse