The best way to go about these questions are to go through the dichotomous key thoroughly... step by step. Read it through as clearly as you can.
For W: The beak isn't slender, so it cannot be A - it is stout and HEAVY, so you follow the directions and go to 2. The bottom surface of the beak is flat, so you can conclude that the bird is likely Geospiza.
Similarly to W, you go back to 1 and see that the beak is stout and heavy - not short and slender - so you go to 2. The bottom surface of the beak isn't flat, so you see 2b. which says that it is curved which is correct. Go to 3. The lower edge of the upper beak is flat compared to Bird Z so you can say that it is likely Platyspiza.
