DOS filename extensions: Which extension commonly indicates that a file is a backup copy created by an application or utility?

Introduction / Context:
Many DOS and Windows utilities create backup copies of files before overwriting or updating them. Recognizing standard backup extensions helps you restore previous versions and manage files safely during maintenance tasks.

Given Data / Assumptions:

• Legacy and many modern tools save backups in the same directory.
• Extensions vary by tool, but .BAK has long been a convention.
• We differentiate .BAK from other common extensions.

Concept / Approach:
The .BAK extension is traditionally used for backup copies of configuration files, documents, and program data. Other extensions here serve different purposes: .BAS indicates a BASIC source file, .COM is a binary executable (command file), and .TXT is a plain text file. While tools may also use .OLD, .TMP, or timestamped names, .BAK remains a widely recognized backup convention.

Step-by-Step Solution:

Verification / Alternative check:
Inspect directories after running editors or setup utilities; backup artifacts commonly appear with .BAK or .OLD.

Why Other Options Are Wrong:
BAS: BASIC source code. COM: executable command file. TXT: plain text, not specifically backup. None: incorrect since .BAK is standard.

Common Pitfalls:
Assuming backups always use the same extension—tools vary; forgetting to check timestamps or original filenames when restoring.

Final Answer:
BAK