Final answer:
The correct reason for statement 5 in the mathematical proof is the Commutative Property of Addition, which allows the rearrangement of terms when they are being added.
Step-by-step explanation:
The question asks to identify the correct reason for statement 5 in the given Proof. The statement transitions from 5y + 10 + 6y to 5y + 6y + 10. The correct reasoning for this change is the Commutative Property of Addition, which states that A+B=B+A. This property is true for the addition of ordinary numbers, such as 2 + 3 giving the same result as 3 + 2. In the context of variable terms in algebra, this property allows us to change the order in which we add terms together without affecting the sum.