Step 1: Arrange by increasing age:
- Children: Youngest, typically 0–12 years.
- Young people: Adolescents/young adults, ~13–25 years.
- Adults: Mature individuals, ~26–60 years.
- Old people: Elderly, typically >60 years.
Step 2: Order: (b) Children, (d) Young people, (a) Adults, (c) Old people.