In the following question, out of the four alternatives, select the option which best expresses the meaning of the idiom / phrase 'Will-o-the-wisp' as used in English.

Introduction / Context:
This idiom question focuses on the colourful expression 'Will-o-the-wisp', sometimes written as 'will-o'-the-wisp'. It originates from mysterious flickering lights seen over marshes, which seem to appear and disappear and lure travellers away from their path. In modern English, the idiom has a figurative meaning related to chasing something that always remains out of reach.

Given Data / Assumptions:
- Idiom: 'Will-o-the-wisp'.
- Options: (a) Something that is impossible to get or achieve; (b) To keep off an unwanted and undesirable person; (c) To spend recklessly; (d) Accept or leave the offer.
- Only one option must reflect the established idiomatic meaning in standard English.

Concept / Approach:
A 'will-o-the-wisp' is traditionally a ghostly, shifting light over marshland that seems to guide travellers and then vanishes, causing them to lose their way. As an idiom, it refers to a goal, hope or dream that is impossible to reach or that constantly escapes one's grasp. It is a symbol of something alluring but unattainable. When examining options, look for the one that conveys this idea of an elusive, unreachable object or aim, rather than unrelated actions like spending or rejecting people.

Step-by-Step Solution:
Step 1: Recall the core meaning: 'Will-o-the-wisp' represents something attractive but impossible or nearly impossible to obtain. Step 2: Option (a) describes 'Something that is impossible to get or achieve', which directly matches the idiomatic sense. Step 3: Option (b) 'To keep off an unwanted and undesirable person' relates to avoiding someone, which has no connection to an elusive goal. Step 4: Option (c) 'To spend recklessly' refers to careless expenditure; this is a completely different idiom (such as 'throw money around'). Step 5: Option (d) 'Accept or leave the offer' describes a take it or leave it situation; again, this does not involve chasing an unreachable dream.

Verification / Alternative check:
Consider a sentence like 'For many young players, the idea of instant fame is a will-o-the-wisp.' Here the phrase clearly means that instant fame is something that seems attractive but is not really attainable or realistic. Replacing it with option (a), we get 'the idea of instant fame is something that is impossible to get or achieve', which preserves the meaning. None of the other options fit when substituted in such real world examples.

Why Other Options Are Wrong:
Option (b) is wrong because it describes keeping away a person, which has no relation to the image of a misleading light or unreachable goal. Option (c) is wrong because reckless spending is not about chasing something unreachable, but about using money carelessly. Option (d) is wrong because it describes a simple two choice decision, not a continuing, fruitless pursuit.

Common Pitfalls:
Students sometimes misinterpret the idiom as something to do with 'ghosts' or 'spirits' because of its origin and may associate it with fear or bad luck. While the origin involves mysterious lights, the established figurative meaning in modern English focuses on the idea of an elusive or unattainable objective. To remember it easily, think of someone chasing a moving light that always stays just out of reach, symbolising an impossible goal.

Final Answer:
'Will-o-the-wisp' means something that is impossible to get or achieve.