If you see each options A,B,C are same only i.e
A) ¬∀x( Graph(x)⟹ Connected(x))
= ¬∀x( ~Graph(x) ∨ Connected(x))
= ∃x( Graph(x)∧¬ Connected(x))
B) ∃x( Graph(x)∧¬ Connected(x))
C) ¬∀x(¬ Graph(x)∨ Connected(x))
= ∃x( Graph(x)∧¬ Connected(x))
So obviously D will be correct.