The code that cannot be parallelized runs for 10ns.
For every additional core, additional 10ns overhead is added to total execution time, overhead for n cores is $(n-1)10$ns.
The code that can be parallelized runs for 90ns on one core, it runs for $\frac{90}{n}$ns on n cores.
Let t(n) be the execution time, it is given by -
$t(n) = 10 + (n-1)10 + \frac{90}{n} = 10n + \frac{90}{n}$
$t(1) = 100, t(2) = 65, t(3) = 60, t(4) = 62.5, t(5) = 68, ...$
Answer - 3