The correct answer to the question is : B) Atoms in stars begin to combine.
Step-by-step explanation:
Before coming into any conclusion, first we have to understand nuclear fusion.
A nuclear fusion is a thermonuclear reaction in which light unstable nuclei will fuse with each other to form a heavy stable nuclei with the release of energy.
It can be easily understood by hydrogen bomb or energy formation process in stars in which four hydrogen nuclei will combine with each other to form a helium nucleus. It is explained as below -
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

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Hence, the correct answer to the question is that atoms combine to initiate nuclear fusion.