ER model doubt

Question

How is it opt D ?

Answer 1

A --> B 
(A)⁺ = {A,B} i.e. A is the key and therefore for any value of A, we can determine B.

B --> A 
(B)⁺ = {B,A} i.e. B is the key and therefore for any value of B, we can determine A.

Therefore for every unique value of A there is a unique value of B and for every unique value of B there is a unique value of A. It can be possible only in One to One relation.
 

Answer 2

For the sake of better understanding, I devised a way for myself!