Remark
On the right, the deposits will build up and the river will have trouble depositing more material and still follow the path it is on. Eventually the river will want to take a short cut across the narrowest part or the neck.
Usually during flooding the water will cut across the narrowest part and an oxbow lake is formed. I think you are intended to answer B, but 50 years may not be enough time in which case D is the answer.
If you have notes on this event, find out what they say. I will choose B, but if you agree don't be surprised if it is D.