In this one-word substitution question, choose the single English word that best expresses the phrase "to renounce one's faith or religion", that is, to formally give up or abandon a previously held belief.

Introduction / Context:
This question tests your knowledge of a specialised term used in religious, political, and ideological discussions. Many languages, including English, have specific words for abandoning one's former beliefs or faith. Understanding such terms is important for reading history, philosophy, religious studies, and current affairs articles with precision.

Given Data / Assumptions:
We are given the phrase "to renounce one's faith or religion". We must select the correct one-word substitution for this act. We assume standard dictionary meanings, with no special technical twist.

Concept / Approach:
The correct word is "apostasy". Apostasy refers to the formal abandonment or renunciation of a religious belief, cause, or political party. A person who commits apostasy is called an "apostate". The core idea is turning away from a belief system one previously accepted. The other options either name unrelated concepts or have very different meanings.

Step-by-Step Solution:
Step 1: Focus on the key action in the phrase: renouncing or giving up a faith. Step 2: Recall that "apostasy" is defined as the act of abandoning a religious or political belief. Step 3: Check each option to see which one matches this definition. Step 4: Confirm that only "apostasy" has this precise sense. Step 5: Select Option C as the correct answer.

Verification / Alternative check:
To verify, consider sentences such as "He was accused of apostasy for leaving the faith" or "Apostasy was punished severely in ancient societies." In each case, the act described is deliberate renunciation. This aligns perfectly with the phrase in the stem. None of the other options would fit naturally into these sentences, confirming that "apostasy" is the right choice.

Why Other Options Are Wrong:
Option A, "Fantasy", refers to imagination or unreal, fanciful ideas; it has nothing to do with abandoning a belief. Option B, "Euphrasy", is another name for eyebright, a plant used in herbal medicine, and is not related to religion. Option D, "Epistasy", is a technical term used in genetics, referring to the interaction of genes, again unrelated to faith or renunciation. Therefore, these options cannot correctly substitute the phrase in the question.

Common Pitfalls:
Candidates sometimes confuse "apostasy" with "apology" or "apostle" because of similar spelling. An apostle is a messenger or missionary, almost the opposite of someone who renounces faith. Another trap is choosing a word purely because it sounds sophisticated, without recalling its meaning. To strengthen vocabulary in this area, it is useful to group words such as "apostasy", "heresy", "orthodoxy", and "dogma" and study them together.

Final Answer:
The correct answer is "Apostasy".