A simpler way to see this is to think about how each hardness tester actually gives you a reading. Brinell and Vickers both leave an impression that has to be measured optically, either the ball indent diameter or the pyramid diagonal, and then plugged into a formula to get the hardness number. Rockwell skips all of that. It presses in a minor load first, then a major load, then releases the major load and reads how much deeper the indenter has sunk compared to the minor load reference. That leftover depth is converted straight into a dial or digital reading. So Rockwell hardness is essentially a depth measurement, which points to option (A).