The hardness of a mineral is measured using the Mohs scale of mineral hardness. On this scale, apatite has a hardness of 5 and calcite has a hardness of 3.
If a mineral can be scratched by apatite (hardness 5) but not by calcite (hardness 3), its hardness must be greater than 3 but less than or equal to 5.
The only possible hardness value from your options that fits this description is 4.
Therefore, the answer is:
4