Step 1: Think about what "optical" actually signals.
The word optical in optical microscope is a direct clue, it tells us the instrument works with visible light and glass lenses, in exactly the same way our eyes see the everyday world, rather than with any exotic radiation.
Step 2: Contrast it with the other radiation types listed.
X rays and gamma rays are extremely high energy electromagnetic radiation used for very different purposes such as crystallography or radiography, and an electron beam is what illuminates a sample in an electron microscope, not an optical one, since electrons have a much shorter wavelength that allows far higher resolution.
Step 3: Settle on visible light.
Early microscopes literally used mirrors to catch sunlight and direct it up through the specimen, and even modern optical microscopes with built in lamps are simply using an artificial source of the same visible spectrum, so among the choices given, natural light correctly represents this visible light illumination.
\[ \boxed{\text{Natural light}} \]