From the expression:
[tex]12+6-9[/tex]
Notice that it has three terms: positive 12, positive 6 and negative 9.
For the commutative property, those terms can be arranged in a different order but the expression will be equivalent. Just be careful not to change the sign of the terms.
From the options, the only one which keeps the term 9 negative and the oters positive, is option C.
