Group Set Swap, Tourney to Claris
That chainring doesn't look particularly worn from this angle. The wear would make the teeth look less symmetrical and more like shark fins. That shorter tooth allows the chain to leave that ring more easily when you change gear.

Ensure the chain is not worn out using a gauge (see http://www.bikeradar.com/road/gear/article/bicycle-chain-wear-explained-46015/ for what types of gauge are available and how to use them). If you grab the chain at the front of the chainring and pull it away from the crank spindle, you shouldn't be able to get more than about 1mm of gap between the chain and the dips between the teeth.