Final answer:
When a 10-year-old child declines to participate in a research study requiring assent, the investigator must respect the child's decision and cannot include the child in the study.
Step-by-step explanation:
If a 10-year-old child firmly declines to participate in a federally funded research study that requires assent for involvement, the investigator must respect the child's decision not to participate. In research involving minors, assent is the agreement of someone not of the age of majority and is distinct from consent, which is legally recognized and typically provided by the parent or guardian. The investigator must honor the assent process just as they would with informed consent, ensuring that participation is voluntary and that the child's choice is respected without penalty or negative consequences.