The correct option is C Advocate
Option C: 'Advocate' is 'a person who publicly supports or recommends a particular cause or policy'.
It means the same as the given phrase and hence can replace them. So, option C is correct.
Option A: 'Pioneer' is 'a person who is among the first to research and develop a new area of knowledge or activity'.
Option B: 'Adviser' is 'a person who gives advice in a particular field'.
Option D: 'Ideologist' is 'a person who develops or supports a particular school of thought'.
These words do not mean the same as the given phrase. Hence they aren't suitable to replace and options A, B and D are incorrect.