Reptilian ancestry of birds can be indicated by

The correct option is scales on their hindlimb.

To understand the evolutionary connection between reptiles and birds, we can examine their anatomical and physiological traits:
1. Scales on their hindlimbs: Reptiles are characteristically covered in epidermal scales that help prevent water loss. Birds possess homologous epidermal scales specifically on their legs (hindlimbs) and toes. This structural similarity is a direct anatomical remnant reflecting their shared evolutionary ancestry with reptilian ancestors (specifically theropod dinosaurs).
2. Eggs with calcareous shell: While both birds and reptiles lay shelled eggs, this trait is also shared with some other groups (like monotreme mammals) and is a broader amniotic characteristic, though still relevant. However, the presence of localized scales is a much more direct, specific morphological signature of reptilian heritage.
3. Four-chambered heart: Most modern reptiles (except crocodilians) have a three-chambered heart, whereas birds have a fully partitioned four-chambered heart. The four-chambered heart in birds and mammals is an example of convergent evolution for high metabolic demands, rather than a direct indicator of typical reptilian ancestry.
4. Presence of crop and gizzard: These are specialized chambers of the digestive tract. While some dinosaurs and modern birds have them, they are specialized adaptations for feeding habits rather than a primary taxonomic link to general reptiles.

Therefore, the presence of epidermal scales on the hindlimbs of birds serves as a prominent evolutionary link indicating their reptilian ancestry.