East-west data traffic in a data center refers to the traffic that flows laterally within the data center, typically between servers within the same data center. This type of traffic usually flows between top-of-rack (ToR) switches, which connect multiple servers or devices in the same rack to each other or to other racks without necessarily routing through the core or distribution layers of the network architecture. This is opposed to north-south traffic, which typically flows in and out of the data center to the wider network or internet.
Therefore, the correct answer is "top-of-rack switches".