Because, the phrase 'of mobility' clearly calls for the use of 'means'. The word approach usually requires the preposition 'to'.
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0915reg
I suppose either "approach" or "means" has the same meaning "measure" here in this sentence.
In your original sentence "means' is closest in meaning to 'way' or 'instrument' but 'means' best fits the sentence. In my sentence approach means 'a set of (proposed) measures or proposals' .
0915reg1st, grammatical error: "approach" needs the preposition "to"Generally speaking, a grammatical error is a syntax error, so there's something wrong with the way you are thinking this through. I think you mean semantic error -- an error regarding the meaning of the word.
2nd, syntax error
0915regMy question is, why the answer "approach" is incorrect?It's incorrect because an approach is a way of going about solving a problem or performing a task. So the word approach is used completely differently from means. approach has a meaning that makes no sense in that blank space.