Step 1: Read the phrase carefully.
The phrase talks about a person who has excessive, burning zeal for a cause, and that cause is specifically religious or political in nature.
Step 2: Match the meaning to a single word.
The word "fanatic" describes exactly this kind of person, someone whose enthusiasm or devotion to a religious or political belief goes beyond reason and becomes extreme or uncritical.
Step 3: Rule out the other choices.
A "lunatic" is someone considered mentally unstable, with no necessary link to a cause. A "critic" is a person who judges or evaluates work, ideas, or performances. A "cynic" doubts the sincerity or motives behind people's actions. A "skeptic" withholds belief until presented with solid evidence. None of these carry the sense of excessive zeal for a cause that the phrase demands.
Step 4: Pick the answer.
Only "fanatic" fits precisely, so option (c) is correct.