Hello there Jonnromeroo, thank you for asking a question here on Brainly!
You Asked ➤ Which of the following processes begins when a star enters the main sequence?
↴ OUR CHOICES ↴
A: Nuclear fission
B: Nuclear fusion
C: Condensation of a nebula
D: Appearance of a supernova
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Answer ➤ B: Nuclear fusion
Explanation: When a star begins the process of being created, it is hot and dense. Really hot and dense! It begins to start fusing with helium which is the start of its main sequence called nuclear fusion.
I hope this answer is helpful to you, but please let me know if you have any other questions regarding it.
<em>- Qamar </em>