Only left aorta or systemic arch is present in
(a) Aves
(b) Mammals
(c) Both aves and mammals
(d) Reptiles

Hint: The animals belong to the class where the presence of hair is one of the unique features and enucleate RBC and absence of renal portal system is observed.

Complete answer:
Four chambered hearts are present in Aves and Mammals where oxygenated blood and deoxygenated blood are completely separated. In aves sinus venosus and conus arteriosus are absent and they have only right systemic arch whereas in mammals they have sinus venosus and conus arteriosus are present and only left systemic arch is present.

Additional information:
- The body of mammals is divided into head, neck, trunk, and tail and it is covered by hair which is one of the unique characteristics of mammals but in mammals like whales and armadillos hair is reduced but hair is present at least in some stage of mammals.
- Sweat glands, scent glands, and sebaceous glands are present and mammary glands are modified sweat glands
- Dicondylic skull, seven cervical vertebrae, and double-headed ribs are present
- Lungs are the respiratory organs and kidneys are the main excretory organs.
- The codont, heterodont, and diphyodont dentition is seen.
- Four-chambered heart with sinus venosus and conus arteriosus. Only a systemic arch is present and the renal portal system is absent. RBC are enucleated, biconcave, and circular
So, the correct answer is '(b) Mammals'.

Note: All the Mammals contain seven cervical vertebrae but in Choloepus and Trichechus only 6 cervical vertebrae are present and in Bradypus 9 cervical vertebrae are seen.
Till now only three pairs of salivary glands are seen but recently the fourth pair of salivary glands are added. RBCs are enucleated except in camels, nucleated RBCs are seen.