Answer: C 
D57.212 is the appropriate code here because it is used for sickle-cell diseases with splenic sequestration which is the diagnosis in this patient. 
Option A is not correct because D57.3 is used to code for sickle cell disease trait and not the disease. 
Option B is not correct because D57.411 is used to code sickle-cell thalassemia, unspecified, with acute chest syndrome. 
Option D is not correct because D57.1 is used to code for sickle-cell disease without a crisis.