Final answer:
The length of side AC is x. Hence, A) is correct.
Step-by-step explanation:
The length of side AC can be determined by using the given information that side AB has a length of x and that the triangle is an isosceles triangle.
In an isosceles triangle, the two equal sides are opposite the two equal angles. Therefore, side AC must also have a length of x. So, the answer is option A) x.