2 carbon atoms which leave in the form of CO2 in TCA, are derived from:

2 molecules of CO2 which are lost in TCA, are from Oxaloacetate. Acetyl CoA in one turn of the cycle gives it's carbons to oxaloacetate, not CO2. In the 2nd turn of the cycle , these two carbons from Oxaloacetate will be lost in the form of CO2. Answer oxaloacetate >>Acetyl CoA.