smc

Note that if we use any pennies whatsoever (one, two, three, or four of them), then the total amount of money would be nonzero mod $5$; as all five options are divisible by $5$, we conclude that we may not use any pennies. Hence the smallest coin we can use is a nickel, and thus the smallest amount we can get is $4\cdot 5=20$. Therefore the option that is not reachable is $\boxed{15}$ $\Rightarrow$ $(A)$. We can verify that we can indeed get the other ones: $25=10+5+5+5$ $35=10+10+10+5$ $45=25+10+5+5$ $55=25+10+10+10$