Here we assumed that company can produce any of the three product in their production  capacity.

Now if company produce X

Then it cannot produce Y and Z

Higher contribution in Y and Z is 2000*3(From Y)= 6000, which is OC

As company will produce higher of production capacity or Market Demand

Now if co. produce Y

Then it can't produce X and Z

Higher contribution in X and Z is 2000*4 (From X)= 8000, which is OC

same way if company produce product Z

Then OC= 2000*4 (From X)= 8000