Database Normalization #5

Consider the following relation and determinants.

R(a, b,c,d)

                   a,c -> b,d
                   a,d -> b
        Also, a,b is a primary key for the above relation.

The above relation is in x NF form where x may take the following values {1,2,3,3.5} corresponding to {1NF, 2NF, 3NF and BCNF} respectively.
What is the maximum possible value of x such that the above relation satisfies the *x*NF form?
Your answer should only be restricted to one of these numbers:1/2/3/3.5 Do not leave any leading or trailing spaces.