Which is the first flying animals on earth
A) Insects
B) Birds
C) Reptiles
D) Mammals

Hint: The only animals that can truly fly are bats, birds, and insects. Other animals manage to travel through the air bike gliding either from great heights or leaping from the depths. The characteristics of the line appear to have evolved separately in history those are in insects, birds, bats, and pterosaurs.

Complete step by step answer:A) Insects - the earliest flying animal on Earth were insects that belong to the largest class of Arthropods. They have an organ-system level of organization. They are bilaterally symmetrical, triploblastic, segmented, and coelomate animals. The body of arthropods is covered by a chitinous exoskeleton.
B) Birds-After insect birds were the second animal on earth that could fly. They belong to the class Aves. They possess beak. The forelimbs are modified into wings. The Hind limbs generally have scales and are modified for walking, swimming, or clasping the tree branches. Kidneys drive without glands except for oil glands at the base of the tail. Endoskeleton is fully ossified and the long bones are hollow with air cavities.
C) Reptiles- the class name refers to their creeping for crawling mode of locomotion. They are mostly terrestrial animals and their bodies are covered by dry and cornified skin, epidermal scales they don't have any external ears opening. The tympanum represents the ear.
D) Mammals - they found in a variety of habitats-polar ice caps, desert, mountains, forest, grassland, at dark caves. Some of them have adapted to fly or live in water. The most unique mammalian characteristic is the presence of the milk-producing gland (mammary glands) by which the young ones are nourished.
Hence the correct answer is option A.

Note: the earliest flying animal was insects. The earliest known insects from the Fossil record are Rhyniognatha which is around 400 million years old. The Fossil was relatively derived within early insects, sharing many characteristics with a winged insect.