Consider a network with $6$ routers $R1$ to $R6$ connected with links having weights as shown in the following diagram.
Suppose the weights of all unused links are changed to $2$ and the distance vector algorithm is used again until all routing tables stabilize. How many links will now remain unused?
First we need to find which are the unused links in the graph
For that we need not make distance vector tables,
We can do this by simply looking into the graph or else DVT can also give the answer.
So, $R1-R2$ and $R4-R6$ will remain unused.
Now If We changed the unused links to value $2$.
$R5-R6$ will Now remain unused.
So, the correct answer is option B).
Only one link is not used
The links R1-R2 and R4-R6 will never be used for data transfer because there are shorter paths available in any case.
If those two link weights are changed to 2, now only one link ie R5-R6 will never be used.
Ans is B