The correct answer is: C. nodes in different data centers.
The bandwidth required for data transfer between nodes depends on the distance between the nodes. The further apart the nodes are, the more bandwidth is required. This is because the data has to travel further, and it takes longer for it to reach its destination.
In option A, the nodes are on the same rack. This means that they are very close together, and so very little bandwidth is required to transfer data between them.
In option B, the nodes are on different racks in the same data center. This means that they are further apart than the nodes in option A, but they are still relatively close together. Therefore, more bandwidth is required to transfer data between them than in option A, but less than in option C.
In option C, the nodes are in different data centers. This means that they are very far apart, and so a lot of bandwidth is required to transfer data between them.
In option D, the data is on the same node. This means that the data does not have to travel anywhere, and so no bandwidth is required to transfer it.
Therefore, the correct answer is C. nodes in different data centers.