What is Cygwin?

A.
Cygwin isa X Windows GUI subsytem that runs on top of Linux GNOME environment

B.
Cygwin is a free C++ compiler that runs on Windows

C.
Cygwin is a freeUnix subsystem that runs on top of Windows

D.
Cygwin is a free Windows subsystem that runs on top of Linux

Explanation:
Cygwin is a Linux-like environment for Windows. It consists of two parts:
A DLL (cygwin1.dll) which acts as a Linux API emulation layer providing substantial Linux API functionality.
A collection of tools which provide Linux look and feel. The Cygwin DLL works with all non-beta , non “release candidate”, ix86 32 bit versions of Windows since Windows 95, with the exception of Windows CE.