The correct option is C, Essential.
The definition of core is the central or most important part of something or something therefore considered necessary.
The definition of essential is something absolutely necessary; extremely important. Therefore the closest synonym to core is essential :)
Wow! This the first time a heard such an intelligent fact
One thing she could do is set up a good sleep schedule. daily exercise is also very important. She also needs to relax, and have fun from time to time. Finally, it's important to concentrate your energy and passionate on a few things, where you can give 100 percent every day, instead of doing many things at a mediocre level.
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