A cylinder would seem to fit the given description that has circular parallel bases and a rounded body.
A triangle with a circular base
A parallelogram, but only if you are counting in base 9.
A shape that has two parallel lines and one perpendicular line is a right-angled triangle. In this triangle, one of the sides can be considered the base with the other being the height, which is perpendicular to the base. The two parallel lines can be imagined as the two sides of a right-angled triangle that are extended. Alternatively, this configuration can also represent a trapezoid if the opposite sides are parallel and one side is perpendicular to the base.
A cone would fit the given description
A cylinder has a circular cross section that is parallel to its base.
A triangle with a circular base
A parallelogram, but only if you are counting in base 9.
No, it is not. For a cone, think of a standard ice-cream cone shape (or a witches hat) - a circular top OR base (but not both), which has sides coming off that taper into a point. Whereas a cylinder has a circular top AND base, (like the shape of baked bean cans), with sides joining both top and base.
A shape that has two parallel lines and one perpendicular line is a right-angled triangle. In this triangle, one of the sides can be considered the base with the other being the height, which is perpendicular to the base. The two parallel lines can be imagined as the two sides of a right-angled triangle that are extended. Alternatively, this configuration can also represent a trapezoid if the opposite sides are parallel and one side is perpendicular to the base.
A cone would fit the given description
A cylinder has a circular cross section that is parallel to its base.
There are infinitely many possible answers. One of these is a icosahedron.
It depends on the base. If the base is an n-sided regular polygon, the pyramid will have n/2 pairs of parallel sides. The number of parallel sides when the base is irregular will depend on the exact details of the base.
a pyramid with the top cut off _____ / \ / \ / \ _________ where top and base are parallel, and sides slope in opposite directions something like that, badly drawn above, for example.
This is an impossible shape. The only 3-d shape with 4 plane faces is a tetrahedron (triangular pyramid). While its base is a triangle, all of its other faces are also triangles and so cannot have parallel sides.
The base of a trapezium is one of the parallel sides. Conventionally it is the longer of the two parallel sides but that need not be the case.
Cone