In the following question, out of the four alternatives, select the one word that is the best substitute for the phrase: the act of speaking irreverently about sacred things.

Introduction / Context:
Single word substitution questions are designed to test how well a learner can match a detailed descriptive phrase with the correct concise term. The phrase here, the act of speaking irreverently about sacred things, comes from religious and ethical vocabulary. English has a specific word that captures insulting or showing contempt for religious beliefs, sacred objects, or deities, and that word appears frequently in exam papers and legal discussions in some countries.

Given Data / Assumptions:

• Descriptive phrase: the act of speaking irreverently about sacred things.
• Options: Atheist, Blasphemy, Bellicose, Defection.
• We need a noun describing an act, not a person, and it must relate directly to religion or sacred subjects.
• Standard dictionary definitions are assumed.

Concept / Approach:
The key clues in the phrase are speaking, irreverently, and sacred things. These indicate disrespectful or insulting speech directed at religious subjects. Blasphemy is precisely defined as impious or irreverent speech or action concerning God or sacred things. Atheist refers to a person who does not believe in God, which is a belief position, not necessarily an act of insulting religion. Bellicose refers to a warlike attitude, and Defection refers to abandoning a duty or allegiance. Thus, only Blasphemy fits all parts of the description.

Step-by-Step Solution:
Step 1: Note that the phrase refers to an act, so the correct answer must be a noun that names an action, not a type of person. Step 2: Identify the core meaning: speaking irreverently about sacred things implies insulting or showing contempt for religious beliefs or holy matters. Step 3: Examine Atheist, which means a person who does not believe in the existence of God. It tells us about belief, not necessarily about an act of disrespectful speech. Step 4: Examine Blasphemy, which is defined as speech or writing that shows contempt for God or sacred things. Step 5: Examine Bellicose, which means warlike or inclined to fight, and is not related to religion or sacred things. Step 6: Examine Defection, which means desertion from a cause, party, or duty, again unrelated to irreverent talk about religion. Step 7: Conclude that Blasphemy is the only word that captures the exact meaning of the phrase.

Verification / Alternative check:
Imagine a sentence from a news report: He was charged with blasphemy for insulting the holy book. Replacing blasphemy with any of the other options leads to nonsense. He was charged with atheist makes no sense because atheist is a person. He was charged with bellicose or defection also do not match the idea of insulting religious beliefs. The description in the question lines up perfectly with Blasphemy as used in legal codes and public debates across different countries.

Why Other Options Are Wrong:
Atheist is wrong because it refers to a person who does not believe in God, which is a belief status rather than an act of irreverent speech. Bellicose is wrong because it deals with aggression and warlike behaviour, not with religion or sacred matters. Defection is wrong because it describes the act of leaving an organisation, army, or cause, usually political or military. None of these words involves speaking irreverently about sacred things.

Common Pitfalls:
Some students confuse Atheist with Blasphemy because both appear in discussions about religion. However, atheism is about belief or lack of belief, whereas blasphemy is about offensive speech or actions. Others may choose Defection simply because it sounds serious or official. The safest strategy is to break the phrase into key ideas and match each part carefully: speech, irreverent, sacred things. Only the word Blasphemy covers all three elements accurately.

Final Answer:
The best single word substitute for the given phrase is Blasphemy.