In this passage about the generation gap, choose the correct word to complete the sentence "It is ______ if ever the most intelligent of apes ever blamed its juniors for their wayward ways."

Introduction / Context:
This cloze passage discusses the generation gap as a special feature of human society. The sentence in question reads "It is ______ if ever the most intelligent of apes ever blamed its juniors for their wayward ways." The writer wants to say that it is very unlikely that animals criticise their younger ones for behaving differently. We must choose a word that expresses this idea of uncertainty or improbability.

Given Data / Assumptions:

The topic is the generation gap among humans as compared to other animals.
The sentence structure "It is ______ if ever..." suggests a comment on whether something has ever happened.
The writer is implying that older animals do not blame younger animals the way older humans blame younger humans.
We assume typical textbook style English for social science style comprehension passages.

Concept / Approach:
The key phrase here is "It is doubtful if ever...". In English, this is a standard pattern used to suggest that something probably never happens. Words like "certain" or "sure" state the opposite meaning. The word "classified" does not fit in meaning at all. We also consider "unlikely", which has a similar meaning to "doubtful", but the classical and most idiomatic expression used in many exam passages is "It is doubtful if ever". Hence, we select the option that best mirrors this stock phrase.

Step-by-Step Solution:
Step 1: Read the sentence: "It is ______ if ever the most intelligent of apes ever blamed its juniors for their wayward ways." Step 2: Understand the intended message. The author wants to emphasise that non human animals probably do not criticise their young generation the way humans do. Step 3: Test each option: "It is doubtful if ever", "It is certain if ever", "It is sure if ever", "It is classified if ever", "It is unlikely if ever". Step 4: "It is doubtful if ever" is a well known expression that means "it is very unlikely that this has ever happened". Step 5: Confirm that "doubtful" fits both the grammatical pattern and the intended meaning of surprise and improbability.

Verification / Alternative check:
We can check the logical flow of the whole paragraph. The passage states that the generation gap is a feature that sets humans apart from other animals. Immediately after that, the author says it is doubtful that even the most intelligent ape has ever blamed its juniors for their strange behaviour. This logic reinforces the idea that only humans have this habit. Among the options, "doubtful" is the only one that expresses this sense while keeping the sentence natural and idiomatic. Although "unlikely" is close in meaning, the traditional phrase used in exam style prose is "It is doubtful if ever".

Why Other Options Are Wrong:

"certain": This would give "It is certain if ever..." which is grammatically and logically wrong. The author is not claiming certainty that apes blame their juniors, but suggesting the opposite.
"sure": Similar to "certain", it expresses strong affirmation. The sentence would then conflict with the main point that animals do not behave like humans in this respect.
"classified": This word relates to categories or secrecy and has nothing to do with the probability of something happening.
"unlikely": While "unlikely" is close in meaning, the specific phrase "It is unlikely if ever" is less natural and less commonly used than "It is doubtful if ever". Exam passages usually copy standard idiomatic patterns, so "doubtful" is the accurate key.

Common Pitfalls:
Candidates sometimes focus only on the dictionary meaning and pick any negative sounding word. They might consider "unlikely" simply because it looks similar in meaning, without thinking about fixed phrases that appear frequently in quality prose. Another pitfall is ignoring the flow of the text and assuming the author is making a strong positive statement, which might lead to choosing "certain" or "sure". Always pay attention to both meaning and collocation when solving cloze passages.

Final Answer:
The correct word to complete the sentence is "doubtful".