The thing to note here is that the argument states "one specific set of parental behaviours toward children always signifies acceptance and a second set always signifies rejection for there is remarkable agreement among investigators about the maternal behaviours." The jump from paternal to maternal means that the father should also exhibit the maternal behaviours indicating acceptance or rejection. This is what B states.

Going through the other options:
A- Interpretation of behaviours is not pertinent to the assumption of this argument
C- Degree of study is irrelevant to the argument. We need to see if fathers behaviours coincides with mothers.
D- Irrelevant
E- Agreement of behaviours is irrelevant to the argument.