Final answer:
Mammals are characterized by having hair or fur, being endothermic vertebrates, producing milk through mammary glands, and typically giving birth to live young, with the exception of monotremes, such as platypus and echidnas, which lay eggs. option a, b, c is correct.
Step-by-step explanation:
The characteristics of mammals that apply from the options provided are: they have hair or fur, give birth to live young (with the exception of monotremes like the platypus and echidnas), produce milk to feed offspring, and are vertebrates. Mammals are endothermic, which means they can regulate their body temperature internally, unlike ectotherms such as reptiles.
While the majority of mammals give birth to live young, there are the egg-laying monotremes that are an exception. Mammary glands are essential for providing milk to their young, and this is a defining characteristic of all mammals. Although not listed in the options, other mammalian characteristics include having a four-chambered heart, sweat glands, and a complex brain structure called the neocortex.
It is false that mammals have only one tooth or that they reproduce both asexually and sexually, as these are not characteristics of mammals.