In the following sentence, a blank has been given. Select the option that best completes the sentence meaningfully: The airline was unable to give us a reason for the ______________ departure delay.

Introduction / Context:
This vocabulary question asks you to choose the adjective that best describes a departure delay for which the airline could not give any reason. The context is about explanation and clarity. When we say there is no satisfactory reason for something, we often use the word inexplicable. Recognising the subtle differences between similar looking words is very important in English language exams.

Given Data / Assumptions:

• Sentence: The airline was unable to give us a reason for the ______________ departure delay.• Options: indivisible, invincible, invisible, inexplicable.• The key idea is that the airline cannot explain why the delay occurred.• We assume normal usage in customer service or travel contexts.

Concept / Approach:
The adjective inexplicable means that something cannot be adequately explained or understood. This is exactly what happens when an airline cannot give a reason for a delay. The other options have meanings that do not fit this context. Indivisible means that something cannot be divided. Invincible means too powerful to be defeated. Invisible means something cannot be seen. Only inexplicable relates to the lack of explanation, so it is the conceptually correct choice.

Step-by-Step Solution:
Step 1: Focus on the phrase was unable to give us a reason, which clearly points to the idea of something that cannot be explained.Step 2: Recall that inexplicable means impossible or very difficult to explain.Step 3: Insert inexplicable into the sentence: The airline was unable to give us a reason for the inexplicable departure delay. This sounds natural and precise.Step 4: Try indivisible. It gives the strange idea that the delay cannot be divided, which is meaningless here.Step 5: Try invincible. That would suggest the delay is some kind of unbeatable enemy, which does not suit the context.Step 6: Try invisible. That would mean the delay cannot be seen, which does not match the idea of lack of explanation.

Verification / Alternative check:
In real life, customer complaints and news reports frequently use the collocation inexplicable delay to describe situations where no one knows the cause. This matches the pattern in the sentence. The other adjectives are used in very different contexts, such as indivisible unity of a nation, invincible army, or invisible ink. These comparisons confirm that only inexplicable belongs with departure delay in this context.

Why Other Options Are Wrong:
indivisible is connected with mathematics or political slogans, not with travel delays. invincible refers to strength that cannot be overcome and thus does not relate to reasons or explanations. invisible is about visibility, not about understanding. None of them convey the idea that no explanation is available for the delay, so all are contextually wrong.

Common Pitfalls:
Because all the options begin with the prefix in, many candidates may rely on guesswork instead of meaning. This is risky. The better strategy is to recall or infer the core meaning of each adjective and then match it with the context. Paying close attention to collocations like reason for the delay strongly guides you toward inexplicable as the only suitable choice.

Final Answer:
The adjective that best completes the sentence is inexplicable.