To use the Commutative Property to determine the missing step in the proof of the equivalence \(12a + 10 + a = 10 + a + 12a\), we should rearrange the terms in the expression.
The missing step can be:
Step 1: \(12a + a + 10 = 10 + a + 12a\)
From here, rearranging the left side using the Commutative Property results in:
Step 2: \(10 + 13a = 10 + 13a\)
So the correct missing step is:
12a + a + 10 = 10 + a + 12a.
Therefore the option to choose is:
10 + 12a + a = 10 + 13a.