Opposite sides are parallel this describes a rectangle, rhombus, square, or parallelogram (there could be more then one answer)

Parallel sides mean that they are always the same distance from each other and will never touch. Knowing that, let's think of the shapes.

Rectangle: Yes, opposite sides are parallel. The two opposite sides of a rectangle will never touch. 

Rhombus: Think of it as a diamond. You can still view it as a rectangle that has the top shifted over a bit. Therefore, yes, opposite sides are parallel because they will never touch.

Square: Is basically the same as the rectangle. Therefore, the answer is yes, opposite sides of squares are parallel.

Parallelogram: The definition of a parallelogram, like in its name, is a four-sided figure with opposite sides parallel. Therefore, yes, parallelograms have opposites sides that are parallel. 

Therefore, the answer is all four of them.