I would say a triangle with two right angles because a triangle's angles add to 180 degrees. If a triangle has two right angles, it means that two angles will be 90 degrees, which add to 180 degrees. A triangle must have three angles and sides, and it is not possible for the last angle to be 0 degrees.
#3 is a triangular pyramid because triangular pyramids have 4 triangular faces. #4 is a cube because there are 6 square faces. #5 is a triangular prism. It has 2 triangular faces and 3 rectangular faces. #6 is a rectangular prism. It has 2 square faces and 4 rectangular faces. (If you don't know what the figure of the net is, resketch it on another piece of paper, cut it out, and form it.)
