the question is only asking for the interest, not how much the invested funds will be after n years. D is just giving you the interest formula. what you need is the formula that will give you the interest earned after n years. since this is simple interest, the principal amount will remain the same at 10k. hence b is correct.

10,000n (x/100)

you can pick numbers in the end just to make sure b is correct.

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Impossible is nothing