A triangle with a circular base
A parallelogram, but only if you are counting in base 9.
A cone would fit the given description
A cylinder has a circular cross section that is parallel to its base.
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 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 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.
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.
Cone
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.
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.
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.