Electron changes their spin to match the spin of proton in the nucleus. Hence, option C is correct.
What is wavelength?
Wavelength is the distance between identical points (adjacent crests) in the adjacent cycles of a waveform signal propagated in space or along a wire.
21-centimetre radiation, electromagnetic radiation of radio wavelength emitted by cold, neutral, interstellar hydrogen atoms. The hydrogen atom is composed of a positively charged particle, the proton, and a negatively charged particle, the electron. These particles have some intrinsic angular momentum called spin.
Hence, option C is correct.
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