Regardless of their frequency, wavelength, or energy, all electromagnetic waves: a. travel only through the vacuum of space. b. are a part of what is called "visible light." c. emit radiation that is harmful to humans. d. travel with the same speed in a vacuum.

All electromagnetic waves travel at the same speed in a vacuum: 3.0 x 10^5 (300,000) kilometres per second. some electromagnetic waves are part of the visible light spectrum and some do emit harmful radiation, but certainly not all. they travel fine on earth without the vacuum of space too.