The key here should be the large-scale migration.
And because of nature of the migration (Datacenter), most likelly is a very-high speed and not all in one shot
So, will be an L2. Here we can exclude MPLS.
They didn't mention anything about DC's location, so in this situation, would exclude CWDM (as it is for metro area only)
So, we have DWDM and MetroE
So, in this question the answer will be really MetroE.
If they would say is more than 10 Gbps, then would be DWDM
This is just my opinion 🙂