The correct answer is: A. Copy file3 file1+file2
The copy command in MS-DOS is used to copy files from one location to another. The syntax for the copy command is:
copy source destination
where source is the name of the file you want to copy and destination is the name of the file you want to copy it to.
In this case, we want to copy the contents of file1 and file2 into a new file named file3. So, the command we would use is:
copy file1+file2 file3
The + sign in the command tells MS-DOS to combine the contents of file1 and file2 into a new file named file3.
Option B is incorrect because it would copy file1 and file2 into file3, but it would not combine the contents of the two files.
Option C is incorrect because it would create a new file named file1+file2+file3, which would contain the contents of all three files.
Option D is incorrect because it would copy file2 into file1+file3, which would not achieve the desired result.