Answer: OPTION D.
Explanation:
By definition, the opposite side of a triangle is that side that is on the opposite direction of an angle.
As you can see, the triangle ABC shown in the figure attached in the problem has the angles: ∠A, ∠B and ∠C.
Then, the angle ∠A is opposite to BC, the angle ∠B is opposite to AC and the angle ∠C is opposite to AB.
Therefore, you can conclude that the answer is the option D, which is: AC.