What characteristic is specific to viviparous animals?

Viviparous animals are specifically characterized by giving birth to live young rather than laying eggs. This reproductive strategy allows the developing embryos to receive nutrients and oxygen directly from the mother, typically through a placenta in many viviparous species. This method of reproduction provides a more supportive environment for the growing young compared to eggs, which can be subject to varying environmental conditions.

While the presence of a placenta is closely associated with viviparity in many mammals, not all viviparous animals possess a placenta, and therefore, having a placenta is not a defining characteristic of viviparous animals as a whole. For instance, certain species of sharks are viviparous and do not utilize a traditional placenta, opting instead for other methods to nourish their young. Thus, while the other options may describe different reproductive strategies or feeding habits, the defining trait of viviparous animals remains their method of birthing live young.