According to Moh's hardness scale, your fingernail would be rated as 2.5. Juan and Carol are trying to identify an unknown mineral. Carol can scratch the mineral sample with her fingernail. Juan and Carol know the mineral MUST be one of these four choices.
Of the choices, the mineral is gypsum. Your fingernail, hardness 2.5, can scratch a mineral with a hardness that is less than 2.5. Of the answer choices, the only possible answer is gypsum.
