Out of the following alternatives, choose the single word that can be used for the given phrase. The phrase is: The act of killing a king.

Introduction / Context:
This question deals with one word substitutions, which are single words that express ideas or phrases that would otherwise require several words. The phrase given here, the act of killing a king, is common in history, politics, and literature. English has a precise term for this specific kind of killing. Recognising such words strengthens vocabulary and allows for more concise and accurate expression, especially in academic and competitive exam settings.

Given Data / Assumptions:

• Phrase: The act of killing a king.
• Options: Regicide, Regalcide, Genocide, Homicide.
• We assume the question refers to the intentional killing of a reigning king or monarch.
• The task is to select the one accurate word that exactly matches this meaning.

Concept / Approach:
Many English words related to types of killing contain the suffix cide, which comes from Latin and means killing or killer. The prefix usually indicates the target. For example, homicide refers to the killing of a human being, suicide to killing oneself, genocide to killing a large group or race, and regicide to killing a king or monarch. The approach is to recall or deduce the meaning of each option by understanding the prefix and matching it with the phrase in the question.

Step-by-Step Solution:
Step 1: Identify the key idea in the phrase: a king is killed. Step 2: Examine Regicide. The prefix regi derives from rex or regis, meaning king, and cide means killing, so regicide literally means killing a king. Step 3: Examine Regalcide. Regal relates to something royal, but this form is not the standard accepted English word for killing a king. Step 4: Examine Genocide. The prefix geno refers to race, tribe, or people, so genocide means killing a race or large group, not specifically a king. Step 5: Examine Homicide. The prefix homo refers to human, so homicide is the killing of a human being in general, not limited to a king. Step 6: Conclude that Regicide is the only option that exactly matches the given phrase.

Verification / Alternative check:
To verify, we can refer to usage in history and political science. Texts discussing the execution or assassination of monarchs, such as accounts of the death of King Charles the First of England, often describe the act as regicide. Dictionaries also define regicide as the killing of a king or the person who kills a king. Other words like genocide and homicide are used in news reports and legal documents with broader targets and do not carry the specific association with a monarch. This confirms that regicide is the correct and precise term.

Why Other Options Are Wrong:
Regalcide: Although built from the root regal, this is not the standard or widely accepted English term for killing a king.
Genocide: This word refers to the deliberate killing of a large group of people, especially those of a particular nation or ethnic group, not specifically a king.
Homicide: This is a general term for killing a human being and is often used in legal contexts, but it is not restricted to kings or monarchs.

Common Pitfalls:
Learners may be tempted by words that sound similar to regal or royal and assume they must be correct. Another pitfall is not understanding the meaning of common prefixes and suffixes like geno, homo, and cide. Building familiarity with these elements helps decode many English words. It is also important to rely on standard usage as found in textbooks and dictionaries rather than inventing or accepting forms that simply sound logical.

Final Answer:
The correct one word substitution for the act of killing a king is Regicide.