Which option best explains the meaning of the idiom "a fool's errand" in standard English usage?

Introduction / Context:
The idiom "a fool's errand" is commonly used in English to describe actions or missions that are pointless or doomed to failure from the beginning. It appears in stories, conversations, and exam questions on idioms. Understanding this phrase helps you interpret situations where someone is being sent or encouraged to do something that is not worth the effort.

Given Data / Assumptions:
- The idiom is "a fool's errand".
- Options suggest meanings such as a blunder, an impossible task, a useless undertaking, or a long journey.
- We must select the explanation that best reflects the established meaning of the idiom.

Concept / Approach:
The word "errand" means a short trip taken to complete some task or deliver a message. When we call something "a fool's errand", we imply that it is pointless, that only a foolish person would attempt it, or that the person is being made to do something which will not bring useful results. It may or may not be absolutely impossible, but it is almost certainly useless or not worth the effort.

Step-by-Step Solution:
Step 1: Break down the phrase: "fool's" suggests that only a fool would undertake this action, and "errand" suggests a task or mission.
Step 2: Connect these meanings to get the idea of a task that is foolish to undertake because it will not bring any real benefit.
Step 3: Look at the options and identify which one captures the idea of being pointless or useless rather than just difficult.
Step 4: Option c, "A useless undertaking that is unlikely to succeed", matches the standard explanation of the idiom.

Verification / Alternative check:
Consider typical usage: "Sending him to convince the manager at this stage is a fool's errand" suggests that the effort will likely fail and is not worth his time. The focus is not strictly on impossibility, but on pointlessness. Someone may still try, but it is seen as foolish. This confirms that "useless undertaking" is the key phrase you should look for in the choices.

Why Other Options Are Wrong:
- A serious mistake or blunder: describes a wrong action but does not capture the idea of a pointless mission.

- An impossible task that no one can ever complete: emphasises impossibility but a fool's errand might be technically possible yet still not worth doing.
- None of these: incorrect because option c clearly fits.
- A long but enjoyable journey: this has nothing to do with foolishness or uselessness; it is about pleasure, not futility.

Common Pitfalls:
Some learners assume that anything containing the word "fool" must mean a simple mistake. However, idioms often have more specific meanings. In this case, focus on the idea that the action is not worth the effort and the person is wasting time or being tricked into doing something unproductive.

Final Answer:
"A fool's errand" means a useless, pointless undertaking that is unlikely to succeed.