The best similarity between birds and mammals is they are both endothermic and have four-chambered hearts.
What are birds and mammals?
Bird comes under class Aves, including organisms such as sparrow, ostrich, kiwi, etc.
Mammals are organisms that contain mammary glands.
Both organisms are endotherms, which means they can regulate their body temperature by themselves.
Both organisms have fully separated four-chambered hearts.
Thus, the correct option is a, They are both endothermic and have four-chambered hearts.
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