Allotropy is a property by virtue of which some chemical elements exist as different physical forms in the same physical state.
For example-Carbon has diamond and graphite as two of its most common allotropes. Both are solids because of differences in the crystal structure have marked distinct properties.
The crystalline structure of diamond is a tetrahedral and that of graphite is sheets. Allotrope is a term used for elements only. Some allotropes may have different molecular formula as Ozone and Oxygen.
So, the correct answer is allotropy.