In this one-word substitution question, choose the term that best replaces the phrase "Government by a king".

Introduction / Context:
One-word substitution questions often focus on forms of government and political systems. Understanding these terms is useful not only for English exams but also for general studies, especially in civics and history. The phrase given here is "Government by a king". You must choose the single word that accurately and specifically describes a political system in which a king or queen is the head of state.

Given Data / Assumptions:

• The descriptive phrase is government by a king.
• The options are autocracy, aristocracy, oligarchy, monarchy.
• The question requires the most precise political term.
• We assume standard political science definitions.

Concept / Approach:
The correct term for a system where a king or queen rules as the head of state is "monarchy". The word is based on Greek roots meaning single ruler. Although an autocracy also involves power in the hands of one person, it does not necessarily require that the ruler be a king; it might be a dictator or another absolute ruler. Aristocracy refers to rule by nobles and oligarchy to rule by a small group. Therefore, monarchy is the most accurate match for government by a king.

Step-by-Step Solution:
Step 1: Recall the basic definition of monarchy as a system in which a monarch, usually a king or queen, is the head of state.Step 2: Match this with the phrase government by a king and see that it fits perfectly.Step 3: Examine autocracy, which means rule by one person with absolute power, but not specifically a hereditary king.Step 4: Examine aristocracy, which is rule by a privileged noble class rather than by a single monarch.Step 5: Examine oligarchy, where a small group of people, often wealthy or powerful, control the government.Step 6: Conclude that monarchy is the only option that directly mentions the idea of a royal ruler and therefore is correct.

Verification / Alternative Check:
Think of well-known countries such as the United Kingdom, where the system is often described as a constitutional monarchy because the monarch is the ceremonial head while elected bodies govern. History books refer to absolute monarchies when kings held all power. We never call these systems autocracies or oligarchies as their primary label, even though some monarchies have autocratic features. This confirms that monarchy is the standard and precise term.

Why Other Options Are Wrong:
Autocracy focuses on absolute power, but that power might be held by a non-royal dictator, military leader or party chief, so it is broader than government by a king. Aristocracy describes rule by a noble class, often hereditary, but not necessarily centralized under a single king. Oligarchy describes rule by a small group, which again could be business elites or party leaders, not a single monarch. These terms highlight who holds power but do not specifically indicate a king as the central figure.

Common Pitfalls:
Students sometimes mix up autocracy and monarchy because both involve strong rulers. The key difference is that monarchy emphasizes royal bloodline and the institution of a king or queen, while autocracy emphasizes concentration of power regardless of background. Similarly, aristocracy and oligarchy may appear in exam options to confuse you, but remembering their roots, "aristo" meaning best and "oligo" meaning few, helps distinguish them. Keeping these definitions clear in your mind makes one-word substitution questions much easier.

Final Answer:
The correct one-word substitution for "Government by a king" is Monarchy.