Final answer:
The missing justification in the proof is the transitive property of equality, which states that if A equals B and B equals C, then A equals C.
Step-by-step explanation:
The missing justification in the proof presented can be determined by examining the types of properties and postulates mentioned in the options. The option that closely relates to the last information presented, which illustrates the commutative property of addition (A+B=B+A), is option b, the transitive property of equality. The transitive property states that if a quantity A is equal to B, and B is equal to C, then A must also be equal to C. This concept is consistent with the other provided examples of mathematical properties, such as the associative, distributive, and commutative properties, which are fundamental to understanding equations and proofs in mathematics.