Which command is used to copy all files from drive a with extension .txt to the currently logged drive and directory?

Copy *.txt a: is the command used in DOS to copy all files with the extension .txt from drive A to the currently logged drive and directory.

The copy command in DOS is used to copy files from one location to another.
*.txt specifies that all files with the .txt extension should be copied.
a: indicates the source drive from where the files should be copied (drive A in this case).
The destination, which is not explicitly shown but assumed, is the currently logged drive and directory.

Copy a. *.txt is not a valid syntax in DOS for copying files; it has incorrect placement of the source and destination.

Copy *.txt c: would copy .txt files to drive C, not to the currently logged drive and directory.

Copy *.txt all.txt would attempt to copy all .txt files into a single file named "all.txt", which is not the same as copying to the currently logged directory.

Therefore, the correct answer is Option B: Copy *.txt a:.