Linear programming - Simplex Method - Stuck with picking the right 'generator'

[Eni]

Dear All,

Thank you for reading my post. I'm stuck with picking the correct 'generator' element in the attached example (Simplex method). As you can see, the solution keeps picking the yellow elements as 'generators', but I don’t understand why we can't choose the purple ones.
Can somebody please explain?

 

[tkhunny]

You can pick whatever you want as long as you're comfortable with a solution that is other then optimal.

There is a two-part rule for picking the generator. Please state it and let's see where the conversation goes.