Which of the following statements is always true for a parallelogram?

a.All sides have equal length and opposite sides are parallel.

b.Adjacent sides have equal length and opposite sides are not parallel.

c.All sides have equal length and opposite sides are not parallel.

d.Opposite sides have equal length and opposite sides are parallel.

d. Opposite sides have equal length and opposite sides are parallel.

The correct answer is option d: Opposite sides have equal length and opposite sides are parallel.