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

The passage lists several reasons for the difficulty in estimating the number of languages: new languages continue to be discovered, there are still unexplored regions, and new languages are often initially mistaken as a dialect of an already known language. However, the passage does not state that people who speak the language do not claim their language. Instead, it mentions that people may be known but the identity of their language may not be, often because they are bilingual or multilingual and converse in lingua franca. Therefore, option (d) is not a reason mentioned in the passage.