Hello Santosh,
may you provide a network diagram and configurations of the hub router?
How is the WAN connected are you using Frame Relay or you are using some form of fractional T1/E1 on the second link?
Or are you using ISDN and the two links are PRI links used for data?
>> Now when the Branch office router C is down or is switched off ,the Link Between A and B is also going down
but you said B is connected on first link alone, do you mean that C and D go down toghether or B and C go down together even if terminated in two different physical interfaces?
Hope to help
Giuseppe