How to get the most recent file using a batch file?

set Path="D:\hello\abc\old"
for /f "tokens=*" %%a in ('dir /A:-D /B /O:-D /S %Path%') do set NEW=%%a&& goto:n 
:n
echo %NEW%

You can use

pushd D:\a
for /f "tokens=*" %%a in ('dir /b /od') do set newest=%%a
copy "%newest%" D:\b
popd