Now when contest is over i can comment why i chose D.

I assumed that birds did land near the picture but they did not peck at grapes,
so i assumed that he was disappointed cause birds weren't scared of boy (boy wasn't realistic enough) and when they landed they saw grapes are also false :)
I imagined this happened: birds see the scene, boy isn't as realistic as he needs to be, they see grapes, they land (because boy isn't real, they aren't afraid), they see grapes aren't real too and they don't peck at them.

Alternative answer would be "what if" situation, what if the birds are part of illusion, so they are part of painting and they cannot peck because they are not real but look real :) .