Bat is a mammal, because it:

Concept: Mammals (class Mammalia) are most reliably defined by one universal trait — every mammal, even the few egg-laying monotremes such as the platypus, nurses its young with milk from mammary glands. Beyond this, most mammals (all except monotremes) also give birth to live young rather than laying eggs. Among the traits commonly used in everyday classification to distinguish mammals from other classes such as birds, reptiles, and insects, giving birth to live young is the reproductive marker most closely and commonly tied to being a mammal.

Application: A bat gives birth to a live baby, matching this widely-used mammalian reproductive marker — among the traits offered here, this is the best-supported reason bats, despite flying like birds or insects, are classified as mammals rather than birds.

Cross-check / Contrast:

• Flying is not exclusive to mammals — insects and birds also fly, so flight cannot be used to classify an animal as a mammal.

• Being nocturnal (active at night) is shared by many non-mammal animals such as owls, moths, and several reptiles, so it does not mark the mammal class either.

• Diet varies widely even among bats themselves (fruit-eating, insect-eating, or blood-feeding species exist) and across other animal classes, so diet type is not a taxonomic criterion for classifying mammals.

Result: Among the choices offered, the trait that makes a bat a mammal is that it gives birth to a live baby — not that it flies, is nocturnal, or is vegetarian.