a) A rhombus with no right angles is one example of a parallelogram that isn't a rectangle
b) A square must have all four sides the same length, whereas a rectangle can have 2 pairs of sides that are different such as a 7 by 8 rectangle.
c) It is possible to have a parallelogram with two sets of different opposite sides, which means that not all four sides are the same length, so it wouldn't be a rhombus. This is similar to part b above, except that the angles don't all have to be 90
d) A parallelogram is a quadrilateral with opposite pairs of parallel sides