Which file extension represents a standard unformatted text document?

The file extension that represents a standard unformatted text document is .txt. This format is specifically designed to contain plain text, without any special formatting, fonts, or styles. It is generally used for simple text editing and can be opened by a wide range of text editors.

The .txt files are readable across different platforms and software, making them versatile for sharing information without the complications of formatting that might not be preserved across different word processing applications. They are typically smaller in size compared to formatted documents because they don't include additional data for styles and layouts.

This makes .txt files a suitable choice for quick notes, programming code, or any situation where simple text representation is needed without additional formatting complexities. The other options denote different types of files: .doc refers to formatted word processing documents, .exe relates to executable files used to run programs, and .zip denotes compressed files used for archiving multiple files into one. Therefore, .txt is the clear choice for an unformatted text document.