Option B is correct
Option D is incorrect as it means Every graph is not connected which is different from Not every graph is connected
Not every graph is connected
It can be rewritten as:
There is a graph which is not connected
so ∃x(graph(x) ⋀ ¬connected(x)
or another way to do it ,simply:
¬∀x(graph(x) → connected(x))
∃x¬(graph(x) → connected(x)) (negation rule)
∃x¬(¬(graph(x) ⋁ connected(x))
And finally
∃x(graph(x) ⋀ ¬connected(x))