It's formal logic:
(A) says that ALL students with disabilities have special needs:
Disab. --> Needs
(D) says that ONLY students with disabilities have special needs:
Needs --> Disab.
The argument says that the school is violating the charter. Why? Because the charter requires students with special ed. needs, yet that school has no learning disabled students.
In a formal logic structure, the author is saying: If you have no students with disabilities, that means you don't have anybody with special ed. needs. (No disab --> No Spec. needs.) The contrapositive of this statement is the correct answer: (D).
Try to see this argument as flawed. Why does the author feel that the school has no special ed students? Because they are none that are learning disabled! As if you have to have a learning disability to have special ed needs -- as if nobody else could have special ed needs. This is not to say that EVERYONE with a learning disability has special ed needs (as (A) suggests), but that ONLY students with a learning disability could possibly have those needs, and noone else.