Which one of the following is NOT the reason for the difficulty in estimating the number of languages of the world?

Correct — D, people who speak the language do not claim their language. The passage never says speakers refuse to claim their language; it says almost the reverse — that many such peoples are bilingual or multilingual and converse with outsiders in a lingua franca, so a distinct ethnic language can go unnoticed. Because option (d) is nowhere given as a reason, it is the correct pick for a 'NOT a reason' question.

Why the others are wrong

• (a)
  New languages continue to be discovered — This IS given as a reason — the passage opens with 'A few new languages do continue to be discovered', so it is a stated cause of the difficulty and cannot be the 'NOT' answer.
• (b)
  There are still unexplored regions — This restates 'as unexplored regions of the world begin to be opened up', a stated reason, so it is not the exception being asked for.
• (c)
  New languages are considered a dialect of a known language — This matches 'similarities... make the investigators assume that what they have found is just a dialect', a stated reason, so it is not the answer.

A 'NOT / except' reading-comprehension item — three options paraphrase reasons actually stated in the passage, and the odd one out, with no support in the text, is the answer.

No directly related past PYQ was found.

• practice — not a real PYQ

  A passage says a bird is hard to count because it migrates, hides in dense forest, and looks like a related species. Which is NOT given as a reason it is hard to count?

  • (a)It migrates
  • (b)It hides in dense forest
  • (c)It resembles a related species
  • (d)It has a very short lifespan
  Answer(d) It has a very short lifespan — the other three are stated in the passage; a short lifespan is never mentioned, so it is the odd one out.
• practice — not a real PYQ

  In a 'NOT / EXCEPT' comprehension question, the correct option is the one that —

  • (a)appears first in the passage
  • (b)is not supported by any line in the passage
  • (c)is the longest option
  • (d)repeats a keyword from the question
  Answer(b) is not supported by any line in the passage — 'NOT'/'except' items ask for the option the text does not back.