Which statement is sometimes true?

a square is a rhombus.

a trapezoid is a parallelogram.

a rectangle is a parallelogram.

a rectangle is a square.

The correct answer is:  [D]:  "A rectangle is a square." 

Note:  A square is a rectangle, but NOT ALL squares are rectangles.

A rectangle is a square.

Step-by-step explanation:

A square is always a rhombus because it satisfies all the properties of rhombus.A trapezoid can never be a parallelogram because in trapezoid there are two non parallel opposite sides but parallelogram has opposite sides parallel.A rectangle is a parallelogram because it satisfies all the properties of parallelogram. A rectangle is a square is sometimes true because a rectangle has opposite sides equal but a square has all its sides equal, thus if a rectangle has all the sides equal then it will be a square.

On the other hand a square is a rectangle because it satisfies all the properties of rectangle.