James thinks that julienne's poor marks are the result of insufficient studying. according to the law of "parsimony", the explanation that is more likely correct is "James's".
The law of parsimony is a critical thinking rule ascribed to William of Ockam who was a philosopher. It directs that a hypothesis ought to give the least complex or simplest conceivable (feasible) clarification for a phenomenon. Like in the given case the simplest explanation for not having good marks is that she did not study enough which is what James said.