I’m still not sure how the answer would be D (CBA), couldn’t it also be ACB?

Answer: No it could not be ACB.
Step-by-step explanation:
Angles are named based on the vertex of the angle so this could be referred to as ∠B. The other way to name an angle is by saying the three points, However, when you say the three points the vertex must be in the middle. Hence, the answer is ∠CBA as the middle point of the angle is in the middle of the name.