This question is kind of poorly worded, because triangles aren't three-dimensional and can't really be "flattened". I assume you're looking for a trapezoid, since if you cut off the "top" of a triangle and draw a line in the space, you get a trapezoid. Nonetheless, if someone else gave you this question, you should let them know that it is poorly worded.
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a triangle
A cone.
A rectangle is not an angle at all. It's a flat shape that looks like a box. A rectangle can never have a reflex angle inside it. But it always has four of them on the outside.
A cylinder is a 3-Dimensional geometric shape that resembles a tube, only a cylinder has no open ends. For instance, if you were to take a paper towl roll and cap off both ends with 2 flat circles, you would end up with a cylinder.
A 3 dimensional triangle is called a pyramid, otherwise a 2D three sided polygon shape is exactly what you called it, a triangle.