In learning the alphabet boxer could not get pass the letters) a-a) b-b) c-c) d-d

D

Explanation:

In Animal Farm by George Orwell, it was stated, " Boxer could not get beyond the letter D. He would trace out A, B, C, D, in the dust with his great hoof, and then would stand staring at the letters with his ears back, sometimes shaking his forelock, trying with all his might to remember what came next and never succeeding.