Answer:
O ad.
Step-by-step explanation:
In the given sentence, the speaker talks of the senator's consecutive policies which gives her the advantage. Considering the frame of the sentence, the best prefix for the blank in the word just before "herents" will be "ad" which makes a new word "adherents" meaning a supporter of a particular party of ideas of someone.
In that sense, adherent will be a supporter of the senator's policies. The other options "ab", "ante" and "post" have no such word with "herents" in the English language. Thus, the correct answer is the second option.