At the bottom of the solution it says -
Note : Labelling the correct triangles is very important for a question like this, as that dictates which sides you’re going to find the ratio of. For instance, if you said that ΔBDA and ΔABC are similar instead, you’ll end up with the incorrect option C - can you see why?
I thought that’s also proportional? is it not the same triangle we look at for the answer:
ΔABC - and the larger-looking of the smaller ones ΔADB