Option B is correct. I and IV are equivalent.
¬∀x(P(x)) = ∃x(¬P(x)) [De morgan's Law]
Alternate approach:
Let's take an example.
Let P(x) = Student x is pass.
I→ Not all students are pass. (which means "Some students are fail")
II→There doesn't exist a student who is pass. (which means "Every student is fail")
III→There doesn't exist a student who is not pass (which means "Every student is pass")
IV→Some students are not pass. (which means "Some students are fail")
I and IV are equivalent.