The correct option is B B
Though they may sound and mean the same thing, using 'on/upon receiving' and 'when you receive' has a slight difference. From the sentence we know that the narrator gives instructions that one must 'count pages' and 'write your registration number on each page'. We use 'on/upon receiving' when the action must be performed Immediately after where as 'when' is used to indicate on the occasion that. So, in the given sentence, 'on receiving the answer sheet' must actually be replaced by 'when you receive the answer sheet'. Therefore, the incorrect option is option (B).