This is the only correct answer.

A Reuters article mentions a miscommunication of the flap setting between PF and PM.
A CNN article says that the PM misheard the PF and selected flap 15 instead of flap 5.
I had to stop reading after the SimpleFlying article.

The interesting part to me is the pilot statements in the NTSB report. They took off at flap 20. At Acc Alt, the PF says that flaps 5 was called. The PM said what was heard was flap 15. The PF said that vmo/mmo moved towards the speed bug instead of moving further away. Now, no matter if the flaps went from 20 (200kts) to 15 (215kts) or 20 to 5 (235kts), the vmo/mmo would have moved away from the speed bug.