An animal has these characteristics: endothermic, 4-chambered heart, amniotic egg. Which taxa could it possibly belong to?

The characteristics described—being endothermic, having a 4-chambered heart, and possessing an amniotic egg—point clearly toward the taxa of Aves (birds) or Mammalia (mammals).

Endothermy is a trait found in both birds and mammals, allowing them to regulate their body temperature internally. The 4-chambered heart, which separates oxygenated and deoxygenated blood efficiently, is a feature that enhances the effectiveness of the circulatory system in both birds and mammals, supporting their high metabolic rates. The amniotic egg provides a protective environment for developing embryos and is a crucial adaptation for life on land, found in both birds and mammals.

In contrast, the other options involve taxa that do not share all these characteristics. For instance, Amphibia typically have a three-chambered heart and do not lay amniotic eggs, while reptiles (which include groups like Chelonia) may exhibit some of these features but are primarily ectothermic and usually have a three-chambered heart, although some have a full septum. These distinctions clarify why the taxa of Aves and Mammalia are the correct identification given the specific traits outlined in the question.