The correct answer is D because micro-partitions in Snowflake are immutable - once created, they cannot be modified in place. This fundamental characteristic is central to Snowflake's architecture, enabling features like Time Travel, efficient metadata management, and reliable data versioning. When data is updated or deleted, Snowflake creates new micro-partitions rather than modifying existing ones. Option A is incorrect because micro-partitions are internal storage units not directly accessible as files. Option B is incorrect because while micro-partitions support metadata for query optimization, they are not indexes themselves. Option C is incorrect because micro-partition sizing is determined by data loading patterns and clustering keys, not Time Travel requirements.