The most appropriate assessment strategy for Joshua's NSSI behaviors would be to use a structured self-report questionnaire specifically designed for assessing NSSI behaviors. So the answer is (c).
This is because a structured self-report questionnaire will provide a standardized and consistent way to assess Joshua's NSSI behaviors. This will allow the clinician to make a more accurate assessment of Joshua's NSSI behaviors and to track his progress over time.
Option (a), conducting an open-ended interview, may not provide enough structure to adequately assess Joshua's NSSI behaviors. Joshua may not be comfortable talking about his NSSI behaviors, or he may not be able to provide all of the relevant information.
Option (b), observing Joshua's behavior discreetly without asking any questions, would not be an appropriate assessment strategy. This is because it would be difficult to get an accurate assessment of Joshua's NSSI behaviors without asking him questions about them.
Option (d), completely ignoring Joshua's self-harming behavior, would be unethical and dangerous. It is important to assess and treat Joshua's NSSI behaviors in order to help him stop harming himself and to improve his overall well-being.