For this case what we must observe are the following triangles: 
 ADB 
 BDC 
 According to the diagram, the triangles have exactly the same dimensions. 
 Therefore, it is fulfilled: 
 

 Then, the value of AC is given by:
 

 Substituting values we have: 
 

 Answer: 
 The value of AC is given by: 
 

 option 3