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

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.