Mario claims that a rhombus is sometimes a square, but a square is always a rhombus. is he correct? explain your answer.

Yes he is a square and a rhombus are exactly the same shape and figure they both have 4 sides and can sometimes have all equal sides :)

That is correct.This is because a square has all the elements a rhombus has.A rhombus has 4 equal sides,just like a square does.And,a rhombus can't always be a square becase a square also contains 4 equal angles,while a rhombus doesn't.