I think is C because in the text it's the main idea.
Precipitation and climate
↪Its a word that you may have to give the most accentuation ↩
↪If you know what the word 'syllable' means then it should give you a hint onto what this word means (stressed syllable), just breakdown the word ↩
The correct sentence is:
Individuals who study the brain understand more about how we learn and think.
"who study the brain" is a relative clause which begins with the relative pronoun "who". It refers to the subject of the sentence "individulas".
The highest one you can get is a Doctoral degree. While it is not needed to practice, you need it to get a license.