Which 02 sensor is this?

I have a 2008 Mitsubishi Galant ES 2.4. The 02 sensor on the front left of the engine's thread has gone bad and even though the part works, the thread will not hold. It pops out when driving and has to be put back into place. We have tried liquid thread and it will not stay for long. So now we are needing a replacement 02 sensor and do not know the exact name of the part to order. I have attached a picture with the location of the 02 sensor that needs replacing in red.