"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.
