Final answer:
Building a tower with building blocks is the most appropriate activity for cognitive development for children 9 months and younger.
Step-by-step explanation:
Cognitive development is an important aspect of a child's growth, especially during the first year of life. For children 9 months and younger, the most appropriate activity for cognitive development would be A) Build a tower with building blocks. This activity helps them develop their hand-eye coordination, spatial awareness, and problem-solving skills.