In hierarchical routing with 4800 routers, what region and cluster sizes should be chosen to minimize the size of the routing table for the three-layer hierarchy?
(clusters- 1) + (regions - 1) + routers
just watch in the order:
routers minimum, then regions minimum, then clusters minimum
so in the first go option C is out of the race
and in the second go, you will get option D with minimum regions.
Thus answer is option D.
Suppose if regions also would have been same in 2 choices, we can consider those two choices further and then compare their clusters to get the best choice for minimum size of the routing table.