Most commonly associated disorder with Down's syndrome among the following is
Hirschsprung disease
CML
Late onset Alzheimer's disease
Early onset Alzheimer's disease
Among the following options, Early onset Alzheimer's disease is most appropriate.

AML & ALL have equal incidence
Duodenal atresia is most Common GI disorder associated with Down's syndrome.
Early onset Alzheimer's disease (Pre-senile Alzheimer's in associated with Down's syndrome.