Answer:
C. 6
Explanation:
ΔADC and ΔACB are similar because two of their angles are congruent (∠ACD and ∠ABC, and ∠DAC and ∠CAB)
Because the sides are similar, you can create a proportion

substitute the given values for the sides. AD=4 and AB=4+5=9

cross multiply

take the square root of both sides
AC=6