Locating toilet tank leak: flapper or flush valve or tanks bolts and washers
If the leak is only from the tank to the bowl, and not on the floor, then the only option of the three you listed that can be the cause is the flapper.
Judging from the photo your flapper can be replaced without any disassembling of the tank.
There are two  flexible tabs on the back of the flapper that go over two fixed pins on the bottom or the  overflow pipe. You can reach down and pull the tabs off of the pins, and reverse the process to install the new one. Very easy.
There are two size's of flappers, 2&quot; and 3&quot;. You can shut of the water supply valve and remove your old one and take it to the store to match if you are unsure of the correct size.