Percentage change is a mathematical concept that can be applied to any quantity that you measure over time. Therefore, D is incorrect, because it can be used in this situation. B is also wrong, because the term "percentage" is perfectly applicable.
The percentage change can be negative, and that means a percentage increase. But the value can also be positive. Therefore, C is incorrect because it can be positive as well. And, as it can be negative if data reflects that, the correct answer is A. The statement makes sense. A percentage change in population can be negative if data reflects that.