Out of the four alternatives, choose the one word that best substitutes the given description. Act of mercy killing.

Introduction:
This one word substitution question focuses on a sensitive concept from ethics and medicine. The description "Act of mercy killing" refers to ending someone's life intentionally to relieve them from severe suffering. You must choose the technical term that captures this idea accurately.

Given Data / Assumptions:
- Description: "Act of mercy killing".
- Options: "Suicide", "Euthanasia", "Immolation", and "Asphyxiation".
- We assume neutral, dictionary based definitions of these terms, without entering into legal or moral debates.

Concept / Approach:
The precise word for mercy killing is "euthanasia". It refers to intentionally ending a patient's life, usually with medical assistance, because of extreme pain, terminal illness, or hopeless condition, with the stated intention of relieving suffering. The other options refer to different kinds of death or killing that do not necessarily involve mercy or relief of suffering in the same sense.

Step-by-Step Solution:
Step 1: Recall that "eu" means "good" in Greek and "thanatos" means "death"; together "euthanasia" literally suggests "good death" or "easy death".Step 2: Connect this with the idea of "mercy killing", where life is ended to remove extreme suffering.Step 3: Examine "suicide". This is the act of killing oneself, not necessarily for mercy, and is self inflicted rather than done by another person.Step 4: Examine "immolation". This usually means killing or sacrificing someone, often by burning, and has religious or ritual associations.Step 5: Examine "asphyxiation". This is death caused by lack of oxygen, for example by choking or suffocation, and is a method of dying, not a term that implies mercy. Therefore, "euthanasia" matches the description best.

Verification / Alternative check:
If you look at discussions in medical ethics, law, or philosophy, "euthanasia" is the standard technical term used for mercy killing. Sentences like "The debate on legalising euthanasia continues" clearly indicate that the word refers to the act of ending life in order to relieve suffering. "Suicide", "immolation", and "asphyxiation" may appear in different contexts but do not carry the specific nuance of compassionate killing for relief of pain.

Why Other Options Are Wrong:
Option A, "Suicide", is the act of taking one's own life, which can happen for many reasons, including depression, shame, or protest, not specifically mercy. Option C, "Immolation", often involves sacrifice or burning, particularly in religious or protest contexts, and again does not inherently imply mercy. Option D, "Asphyxiation", is a medical term for death by suffocation or lack of oxygen, and describes a physical mechanism rather than the moral idea of mercy. Only option B, "Euthanasia", correctly combines the notion of killing with the purpose of relieving suffering.

Common Pitfalls:
Candidates may confuse "suicide" and "euthanasia" because both involve death that is not purely accidental. However, the key difference is that suicide is self inflicted, whereas euthanasia is usually performed or assisted by another person in a medical or controlled context. Another pitfall is choosing "immolation" because of its association with religious sacrifice, but this has no direct connection with the idea of mercy killing in modern ethical discussions. Always focus on the key word "mercy" in the question to guide your choice.

Final Answer:
The correct one word substitute for "Act of mercy killing" is Euthanasia.