The correct option is A a
The option that has an error, is the correct option.
Option A: The error lies in part (a): 'there is not many traffic'. The word 'many' is the error.
'Many' refers to 'a large number' of something; it describes countable nouns, e.g. 'many people were seen causing havoc in the city'.
The noun here 'traffic' is an uncountable noun or a mass noun. So, this word 'many' cannot modify 'traffic'.
The correction is 'There is not much traffic'.
Hence option A is the correct option with the error.
The other parts (b) and (c) do not have any error in them. They are grammatically correct and are complete in meaning.
Hence options B and C are not the right answers.
Option D is incorrect because the sentence has an error (a).