What is the significance of "atomicity" in blockchain transactions, especially for complex DeFi operations involving multiple steps?

A All operations within a transaction either fully succeed together or the entire transaction is reverted, preventing partial execution that could leave the system in an inconsistent state B Atomicity means transactions are processed one atom at a time over several days C Atomicity guarantees a transaction will always fail regardless of its logic D Atomicity has no relevance to smart contracts, only to physical chemistry