While working with MS-DOS which command is used to copy all files with extension .txt into one file named all.txt?

Answer: Option D

To copy all files with the extension .txt into one file named all.txt in MS-DOS, you would use the copy command with the source files wildcard and specify the destination file. Here’s how each option works:
Option A: Copy a: *.txt attempts to copy all .txt files from drive a: but does not specify a destination file.
Option B: Copy *.txt a: attempts to copy all .txt files to drive a: without specifying a destination file name.
Option C: Copy *.txt c: attempts to copy all .txt files to drive c: without specifying a destination file name.
Option D: Copy *.txt all.txt correctly copies all .txt files in the current directory to a single file named all.txt.

Therefore, Option D is the correct command to copy all files with the extension .txt into one file named all.txt in MS-DOS.