A couple ways to approach this question: (Vitamins found starting FA2020 p65)

• Pale, bulky stools with increased fat concentration - This is indicative of fat malabsorption. Even without identifying the child's disorder (likely cystic fibrosis because of this and the lung involvement) you can use this tid-bit of info to narrow the answers to the vitamins that are poorly absorbed in fat malabsorption (i.e. the fat-soluble vitamins ADEK). This would eliminate all but two answers, and being able to realize that vitamin D is associated with Rickets in children (bone issues, NOT neuro) while vitamin E is associated with neurologic issues (because of its antioxidant properties) would lead you to the answer.
• If you did not realize that they were trying to get at fat malabsorption, you can still narrow down the answers if you know the other deficiencies. As mentioned above, vitamin D is rickets/bones so it can be eliminated. Vitamin C deficiency is scurvy which presents with swollen gums, easy bruising, weakened immune response, anemia, etc. none of which include neuro symptoms as described. Niacin (vitamin B3) deficiency leads to pellegra which is associated with the 4 D's - diarrhea, dermatitis, dementia, and death; and this boy does not have dermatologic symptoms and his neurologic symotoms are not characteristic. Biotin (vitamin B7) deficiency is rare and associated with dermatitis, enteritis, and alopecia. Vitamin E associated with ataxia and demyelination of the posterior columns leading to decreased position and vibration sense, so it matches the symptoms.\