"Regular quadrilateral" means it has four sides, and the
sides AND the inside angles are all equal.
-- The one at the upper left is a rhombus. All 4 of its sides
are the same length, and the first time I saw it, I picked it.
But then I remembered that in order to be 'regular', the
inside angles also have to be equal, and in a rhombus,
they're not.
-- The one in the lower right is a square. It has four sides,
they're all equal, and the inside angles are all the same too.
So the square is the only one that does the job for you.
