Similar figures are figures which has the same shape however they do not always have the same size. It should not be used interchangeably with congruent figures since the latter would refer to figures which has the same size and shape. Thus, congruent figures are always similar figures but similar figures are not always congruent figures. From the choices given, I think the best choice would be the first option. The two squares would be the pair of non-congruent figures that are always similar. This is because no matter what are the side lengths of the squares, it will always have the form of the square.
