Final answer:
Mental status is important in objective data for nursing diagnosis because it provides insight into cognitive abilities and mental well-being, indicating cooperation, goal attainment, infection presence, ability to process teaching, and potential for violence.
Step-by-step explanation:
The mental status is an important and measurable item to include in objective data when selecting a nursing diagnosis because it provides insight into the patient's cognitive abilities and overall mental well-being. It helps assess the patient's level of cooperation, their ability to process teaching, and their potential for violent behavior. Additionally, it can indicate if the patient will be able to reach their goals and if there is an infection present.