Which property of mortar allows it to adapt and return to its original form after it has been compressed?

The correct answer is elasticity. Elasticity refers to the ability of a material to deform under stress and then return to its original shape once the stress is removed. In the context of mortar, when it is compressed—whether by settling structures or weight above—it can absorb that pressure and, upon the release of that pressure, revert back to its initial form. This property is crucial for maintaining the integrity of masonry structures, as it helps accommodate movement and stresses that occur over time.

Flexibility, though related, refers more specifically to the ability to bend without breaking and does not necessarily imply a return to the original shape after deformation. Plasticity describes a material's ability to undergo permanent deformation without breaking when subjected to forces, which is not what occurs with mortar since it ideally should return to its original form. Sustainability usually pertains to environmental considerations or the lifecycle impact of materials rather than physical properties related to deformation.