Final answer:
According to Thomas and Chess, the 18-month-old girl would be considered a difficult baby based on temperament traits.
Step-by-step explanation:
According to Thomas and Chess, Brianna, the 18-month-old girl, would be considered a difficult baby. Difficult babies cry frequently, are hard to soothe, and wake up frequently during the night. This classification is based on temperament traits, such as intensity, activity level, adaptability, and regularity, which can vary between individuals.