Bank 1 is in reference to where the #1 cylinder is. on a 4 cylinder engine, there is only bank 1.
On 6 cylinders, the first 3 cylinders are bank 1, the last 3 bank 2. this is because there are 2 separate manifolds on the exhaust.
On v6, v8, v12 engines, where the #1 cylinder has been designated (by said auto manufacturer) will be bank 1.
The pre cat o2 sensors will be designated 'sensor 1' on a given bank, while 'sensor 2' will be post cat sensor on a given bank.
Sensor 1 (pre cat) makes the changes and all the sensor 2 does (post cat) is monitor the changes to make sure the cat is doing the right things.
I'm going to say is the one after the cat or its actually the cat, does it smell eggy? usually thats signs of the cat failing?