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thnx.

D will be correct because see when the LHS and RHS of a router has same bandwidth it will not create congestion.

Ex: Suppose R5 has to transmit full data in R6 direction then  it will create congestion because b<c.
ok, got it thanks
With your logic, same is the case for $R1$, $R2$, $R3$ and $R4$. So answer should be option $a$.

Most important point is that "all the links are full duplex."

Consider the case 1

Suppose hosts h2,h3 and h4 wants to send the data to the host h1.So the data from h3 and h4 will be forwarded by router R2 to R3 and then R3 will forward the data to the router R1.Now suppose the data from h2 also come to the router R1 at the same time.In this case R1 now have total (1+2)=3 Mbps data to send to h1  though the link R1 to h1 which is of 1Mbps.So the extra data (beyond the capacity of the link R1toh1 ) should wait on the router R1 which will lead to the congestion in the router R1.

With similar logic we can conclude that R2 and R6 and R7 can be congested.

Consider the case 2.

Suppose the hosts h3,h4,h7,h8 want to send data to host h1 or h2.Using the above mentioned logic for case 1 we can show the router R3 can be congested(Hints b+c>b).

So using the similar argument we can show the routers R3 and R5 can be congested.

so the final answer will be R4.(in this case c=c)

because R4 is connected to high bandwidth link i.e. 4mbps on both side