Answer:
c. Subtract total satisfaction from consuming N - 1 (first) products from total satisfaction from consuming N products
Step-by-step explanation:
By definition, marginal utility of consuming one more unit of product or service is the additional satisfaction of consuming that unit of product or service.
That additional satisfaction from (consuming) the Nth products = total satisfaction from (consuming) all N products - satisfaction from consuming (first) N - 1 products
(first) should be added, because you are finding the satisfaction from the last consumed product.