Final answer:
The subject of this question is Biology, and it is discussing a term or concept related to child development or infant behavior. One possible answer could be that Brianna's behavior is indicative of 'difficult temperament' according to Thomas and Chess.
Step-by-step explanation:
In this question, the subject being discussed is Biology. The question is asking for the definition of a term or concept related to Biology. The information provided about a baby named Brianna crying frequently, being hard to soothe, and waking frequently during the night suggests that the term or concept in question is related to child development or infant behavior. One possible answer could be that Brianna's behavior is indicative of 'difficult temperament' according to Thomas and Chess, who have conducted research on child temperament.