At the bottom of the solution it says -

Note: Labelling the correct triangles isveryimportant 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 optionC- 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