What should be subtracted to the polynomial x2 − 16x + 30,

We know that, if $x=\alpha$, is zero of a polynomial then $x-\alpha$ is a factor of $f(x)$

Since 15 is zero of the polynomial $f(x)=x^{2}-16 x+30$, therefore $(x-15)$ is a factor of $f(x)$

Now, we divide $f(x)=x^{2}-16 x+30$ by $(x-15)$ we get

Thus we should subtract the remainder 15 from $x^{2}-16 x+30$,

Hence, the correct choice is (c)