In the following question, out of the four alternatives, select the option that best expresses the meaning of the idiom/phrase Achilles' heel, which is often used to describe a particular type of weakness.

Introduction / Context:
This idiom based question tests your knowledge of classical references in English. The phrase Achilles' heel comes from Greek mythology. Achilles was a mighty warrior who was invulnerable except for one spot on his body, his heel. In modern English, this story is used metaphorically to describe a critical weakness in a person, system, or organisation, even when everything else appears strong or impressive. Understanding such idioms is important for reading articles, essays, and competitive exam passages.

Given Data / Assumptions:

• Idiom: Achilles' heel.
• The context of use is usually when someone or something is strong in many ways.
• There is, however, one specific weak point that can cause serious trouble.
• Options describe different ideas such as weakness, secret weapon, hidden deed, or scapegoat.

Concept / Approach:
An idiom does not mean exactly what its individual words suggest. Achilles' heel is a fixed expression that means a vulnerable point or fatal flaw in an otherwise strong character or structure. The key words to look for in the options are weakness and overall strength. Option a directly mentions a fatal weakness in spite of overall strength, matching the mythological story and modern usage. The other options present different concepts: secret weapon, past deed, and scapegoat, which are unrelated to the idea of vulnerability.

Step-by-Step Solution:
Step 1: Recall the myth: Achilles was a great warrior protected from harm except for his heel, which ultimately led to his death. Step 2: Translate the myth into modern usage: Achilles' heel now means a critical weak point that can cause failure. Step 3: Examine option a: A fatal weakness in spite of overall strength. This matches the derived meaning exactly. Step 4: Examine option b: A secret weapon to be used as a last resort. This is about strength and strategy, not weakness. Step 5: Examine option c: A past deed which one is trying to hide. This is more about guilt or secrecy than about vulnerability. Step 6: Examine option d: The person who is blamed if anything goes wrong, which is closer to the idiom scapegoat, not Achilles' heel. Step 7: Conclude that only option a correctly captures the meaning of the idiom.

Verification / Alternative check:
Check the idiom in example sentences: Poor time management is his Achilles' heel or The outdated software is the company's Achilles' heel. In both cases, the phrase refers to a serious weakness that may cause failure. If you substitute option a into the sentence, the meaning remains intact: Poor time management is his fatal weakness despite his overall strength in other areas. None of the other options can be inserted naturally without changing the idea completely.

Why Other Options Are Wrong:
Option b, a secret weapon to be used as a last resort, is about strategic advantage, the opposite of weakness. Option c, a past deed which one is trying to hide, relates to secrecy and shame, not structural weakness. Option d, the person who is blamed if anything goes wrong, defines a scapegoat, which is someone blamed for problems, not the problem itself. Thus, they do not match the idiom Achilles' heel.

Common Pitfalls:
Some learners confuse Achilles' heel with other idioms like trump card (secret weapon) or skeleton in the cupboard (hidden shame). This happens if the student identifies only a vague idea of a problem or secret without focusing on the specific nuance of weakness. To avoid this, always recall the myth or a clear mental image: a powerful hero defeated by one small vulnerable point. That image leads directly to the correct meaning of a fatal weakness in spite of overall strength.

Final Answer:
The idiom Achilles' heel means a fatal weakness in spite of overall strength.