Which of the following statements is a true axiom?

The true axiom among the statements provided is that when A equals C and B equals C, it follows that A equals B because both are equal to C. This principle is rooted in the transitive property of equality, which states that if two values are each equal to a third value, then those two values must also be equal to each other. In this case, since both A and B are equal to C, it logically leads to the conclusion that A must equal B.

The other statements, while they relate to mathematical operations and equality, do not all conform to the same universal principles of equality in the way that the selected statement does. The other choices involve operations (addition, subtraction, multiplication, and division), which require additional contextual understanding of the numbers involved and can lead to misinterpretations if applied without care. However, the transitive nature of equality is a fundamental axiom and is always true in mathematics, making the correct choice evident.