You have only written 1d. 1d indicates he is only going. But since he has to come back too, you have to add another d. Hence, it is N + 2d. It is written it is each way, so you have to add another d.
Differentiate this function. You shall get your answer.
Regarding the second doubt, I had a similar doubt too. But then I realised that I am reading too much into the question by assuming that each unit means discrete goods.
Judging past years questions, the paper setters always made sure to explain that the good is discrete when they explicitly state that consumers buy discrete quantities. Since it is not mentioned here, it will not be the case.
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