In the following sentence, choose the correct word to fill in the blank: "The investigations revealed a ____________ lack of efficiency in the functioning of the airlines."

Introduction / Context:
This blank filling question assesses your vocabulary and your ability to pick a word that collocates naturally with "lack of efficiency" in a formal investigative context. The sentence talks about investigations into airline functioning, suggesting that the deficiency was clear and noticeable rather than hidden.

Given Data / Assumptions:

• Sentence: "The investigations revealed a ____________ lack of efficiency in the functioning of the airlines."
• Options: plain, obscure, conspicuous, concealed.
• The investigations bring something to light.

Concept / Approach:
The key word here is "revealed", which suggests that the investigation uncovered something evident or obvious once examined. A "conspicuous lack" is a standard collocation meaning a very noticeable deficiency. We must choose a word that both fits logically and is commonly used in such formal reports.

Step-by-Step Solution:
Step 1: Understand that the investigation uncovered or highlighted the lack of efficiency. Step 2: Examine "plain". "Plain lack of efficiency" is possible, but the more common collocation is "plain fact" or "plain truth", not "plain lack". It is weaker than required. Step 3: Examine "obscure". An "obscure lack of efficiency" would mean a hidden or not easily noticed lack, which clashes with the idea of being revealed clearly. Step 4: Examine "conspicuous". "Conspicuous" means easily seen or clearly noticeable. A "conspicuous lack of efficiency" matches the idea that inefficiency stood out once investigated. Step 5: Examine "concealed". "Concealed lack of efficiency" would mean hidden, but investigations revealing something usually make what was hidden become conspicuous, not stay concealed. Step 6: Conclude that "conspicuous" is the best fit.

Verification / Alternative check:
Insert "conspicuous" into the sentence: "The investigations revealed a conspicuous lack of efficiency in the functioning of the airlines." This sounds like typical formal reporting language. Substituting "obscure" or "concealed" makes the sentence contradictory, because something that has been revealed is no longer obscure or concealed. "Plain" is grammatically possible but far less idiomatic than "conspicuous lack".

Why Other Options Are Wrong:
"Plain" is not wrong in meaning, but it does not form as strong and natural a collocation as "conspicuous lack" in formal English. "Obscure" means unclear or difficult to understand, which conflicts with the outcome of an investigation that clarifies issues. "Concealed" refers to something hidden, which again conflicts with "revealed".

Common Pitfalls:
Candidates may be tempted by words like "concealed" or "obscure" because they match the idea that problems are often hidden before an investigation. However, the sentence focuses on what the investigation revealed, not on what remained hidden. Also, examining common collocations such as "conspicuous lack", "conspicuous absence", and "conspicuous failure" can guide you to the correct answer quickly in vocabulary questions.

Final Answer:
The correct word to complete the sentence is "conspicuous", giving "a conspicuous lack of efficiency".