Which property states that addends can be rearranged inside and outside of parentheses without affecting the sum

Associative property of addition.

Step-by-Step explanation:

It is clear that the parentheses do not affect the sum; the sum is the same regardless of where the parentheses are placed. For any real numbers a, b, and c, (a + b) + c = a + (b + c). The example below shows how the associative property can be used to simplify expressions with real numbers.

Also we can use this property for algebraic expressions like polynomial expression as well.

Hence, associative property of addition is the answer.