Which of the following forms of electromagnetic waves is least dangerous to the human body?
a. gamma rays
b. ultraviolet rays
c. microwaves
d. x-rays

Starting with the most obviously dangerous, gamma rays are the highest frequency and most energetic form of light. They penetrate the human body and cause radiation poisoning after nuclear events like melt downs. Very harmful.

X-Rays penetrate skin and muscle but not bones. They're useful for medical and dental imaging, but if they interact with your DNA, they can cause cancer. In fact, the first people to die because of X-rays were shoe salesmen. They used an x-ray device to scan people's feet and were overexposed to harmful x-rays.

UV rays can interact with your skin and your eyes. They cause you to tan, get a sunburn, or get skin cancer. Their wavelength allows them to penetrate the top layers of your skin, and if they interact with your skin cells, they can develop cancer.

Visible light is harmful to your eyes when bright enough, like staring at the sun or a high energy laser beam. Dark spots can be burned into your retinas, and you'd have those spots permanently.

Infrared light is heat, so you can be burned when close to fires or radiant heat sources.

Since we are mostly made of water, microwaves can harm us. Microwave ovens are made to be very safe, but when they were first developed, radiation leakage could be harmful. If there was a big enough microwave oven and you were inside it, it would be incredibly harmful!

It is not known if radio waves can be harmful. Our cell phones transmit in the radio/microwave part of the spectrum, and studies are being done to see if they have long term harmful effects.

So while all forms of electromagnetic waves can be harmful, the ones usually though of as harmful are Gamma rays, X-rays, and Ultraviolet light.