Final answer:
The earliest known land animal fossil is of a millipede, one of the first arthropods to transition to a terrestrial lifestyle more than 430 million years ago.
Step-by-step explanation:
The oldest currently known fossil of a land animal is a millipede (C). Arthropods were the first animals to inhabit terrestrial environments, and millipedes were among these early land dwellers, moving to land approximately 430 million years ago. While trilobites are some of the oldest known arthropods, appearing over 500 million years ago, they were marine arthropods and not land-dwellers. Importantly, fossil evidence has also shown early forms of life, such as microbial mats, dating back roughly 3.5 billion years, and sponge-like creatures from 650 million years ago, but these do not pertain to animals living on land.