Answer: C. $\mathbf {17}$
Explanation:
If all the cities are connected in a loop using $17-\text{Links}$, and if even a single link fails the loop can still be traversed using the other path.
$\therefore$ Connecting cities using $16-\text{Links}$ or less would not be sufficient because with just $16$ Links it would become a spanning tree that would consist of an only single path and will get disconnected even if a single link fails.