What is the best description of an EnCase evidence file?

An EnCase evidence file is best described as a bitstream image of a source device written to a file or several file segments. This means it captures all the data from a source storage device in a way that preserves the original data structure, including unused space, slack space, and deleted files. This is crucial for forensic investigations, as it allows examiners to analyze the data without altering the original source, maintaining its integrity for legal processes.

The term "bitstream image" indicates that every bit of data is copied, ensuring a complete and faithful representation of the source. Writing this data to a file or multiple segments allows forensic analysts to work with the image more flexibly, facilitating efficient analysis while keeping the original data intact.

The other descriptions either mischaracterize the nature of the image (like suggesting it’s merely a “mirror image” or "sector-by-sector" without acknowledging the thoroughness of a bitstream capture) or do not specify the use of files, which is a key aspect of how EnCase manages evidence files. Therefore, the correct answer accurately captures the method of creation and structure of an EnCase evidence file.