The term `Dolby B` or `Dolby C` printed on tape recorders and other sound systems refers to types of noise reduction systems. This is linked with option 3, which indicates a noise reduction circuit.

Option 1, frequency modulated system, and option 2, amplitude modulated system, both refer to methods used to encode information on a carrier wave for transmission, commonly used in radio broadcasting. They are not specific to noise reduction in sound systems.

Option 4 suggests both DC and AC power can be used. While it`s a fact that many electronic devices can use both sources of power, it has no relation to `Dolby B` and `Dolby C`. These phrases specifically denote noise reduction technologies and not power sources.

So, option 3 is the correct answer: `Dolby B` and `Dolby C` are indeed a type of noise reduction circuit technology, designed to improve the sound quality of audio systems.