The reasoning is from the following:
The formula for simple rate of interest is F = P(1+rt) where
F = future value
P = present value
r = rate
t = time

We can substitute x for r
n for t
and 10,000 in for P

F = 10,000(1+xn)

This formula will tell you the principal (original amount invested) + interest, but we only want interest, so the "1+" portion of the formula in unecessary. -->
F = 10,000(xn)

Now, we need to get the x into terms of a percentage, so we divide x by 100 -->
F = 10,000(n * x/100)

If we move the n outisde the brackets, we get -->
F = 10,000n(x/100) --> C

Answer: C