In the following sentence, one numbered part may contain a grammatical error, or the sentence may be fully correct. Choose the number of the part that has an error, or choose option (4) for No Error. Sentence: Sometimes it appears as if they are deliberately (1) / limiting their chances of survival in order (2) / to observe some customary or taboo. (3)

Introduction / Context:
This item checks the learner's understanding of correct noun usage and collocations in English. The sentence uses the words customary and taboo in an incorrect combination, and the task is to identify the segment where the grammatical or lexical error lies. Such questions help build sensitivity to how adjectives and nouns should be paired in natural English.

Given Data / Assumptions:

• The sentence is separated into three numbered parts plus a No Error option.
• Part (1) describes the overall situation: it appears as if they are deliberately doing something.
• Part (2) explains the purpose: limiting their chances of survival in order.
• Part (3) provides the object of observe: some customary or taboo.

Concept / Approach:
The main concept here is correct word form and parallel structure. Customary is an adjective, while taboo in this sentence is used as a noun meaning a prohibited practice. When we use some before a noun phrase, we normally expect nouns on both sides of or, for example some custom or taboo. Placing an adjective directly before or, without a noun, breaks the parallel structure. Therefore, the phrase some customary or taboo is grammatically faulty and must be corrected to some custom or taboo or some customary taboo practice. This reveals that part (3) is the incorrect segment.

Step-by-Step Solution:
Step 1: Read the full sentence once to understand the general meaning about people limiting their chances of survival.Step 2: Check part (1): Sometimes it appears as if they are deliberately. The structure is natural and grammatically correct.Step 3: Check part (2): limiting their chances of survival in order. This is correctly formed and leads into an infinitive of purpose, which appears in the next part.Step 4: Examine part (3): to observe some customary or taboo. Here, some should introduce a noun, but customary is an adjective form of custom.Step 5: Notice that taboo is used as a noun, so the structure after some should be noun or noun, giving some custom or taboo or some ritual or taboo, not some customary or taboo.Step 6: Conclude that segment (3) contains the error because it uses an adjective without a noun and breaks the expected parallelism.

Verification / Alternative check:
To verify, replace the faulty phrase with a correct one. If we change part (3) to to observe some custom or taboo, the complete sentence becomes: Sometimes it appears as if they are deliberately limiting their chances of survival in order to observe some custom or taboo. This version reads smoothly and maintains clear meaning. Only a small word form change from customary to custom was needed, confirming that the third part was indeed the incorrect segment. No other part of the sentence demands a structural correction.

Why Other Options Are Wrong:
Option 1: Part (1) uses correct tense and phrasing. The combination Sometimes it appears as if they are deliberately is standard and acceptable.Option 2: Part (2) is grammatically fine. The phrase limiting their chances of survival in order correctly sets up the purpose that follows.Option 4: The sentence is not free from error because of the phrase some customary or taboo, so No Error cannot be the correct answer.

Common Pitfalls:
Many learners read quickly and do not consciously distinguish between custom and customary. They may feel that customary is some type of noun simply because it is familiar. Another pitfall is ignoring parallel structure around or, where the two items being compared or contrasted should share similar grammatical forms. Paying attention to whether words are nouns, adjectives, or verbs is essential when solving error spotting questions of this type.

Final Answer:
The incorrect expression is in the third part, so the correct option is 3.