Final answer:
The true equation among the given options, that demonstrates matrix properties in mathematics, is A + B = B + A, reflecting the commutative property of matrix addition.
Step-by-step explanation:
The subject of this question is the properties of matrix addition in mathematics. Given that A and B are matrices, C is a scalar, and D is a matrix of the same size as A, the equation that is true out of the provided options is A + B = B + A. This equation exemplifies the commutative property of addition, which states that the order of addition does not change the sum. This property applies to the addition of matrices just as it does with the addition of ordinary numbers. Other given options, such as A + B = B + C or A + B = B + D, are incorrect because matrix addition requires the added entities to be of the same dimensions, and a scalar cannot be added to matrices in the same sense as matrix addition. The equation A + B = B - D cannot be true as it suggests that A is equivalent to -D, which does not hold true without additional context.