Use the Commutative Property to determine the missing step in proving the equivalence of 12a+10+a=10+a+12a.

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**.