Final answer:
The transition from (a + c) + x to a + (c + x) is an application of the Associative Property of Addition, indicating that the grouping of addends does not affect the sum.
Step-by-step explanation:
The algebraic property that was applied to the expression (a + c) + x to get the expression a + (c + x) is the Associative Property of Addition. This property states that when adding three or more numbers, the way in which the numbers are grouped does not change the sum. In other words, regardless of how the numbers are associated or grouped in pairs, the total sum remains the same. Therefore, (a + c) + x results in the same value as a + (c + x), which demonstrates this property.