Concept:The rate of a reaction increases with larger surface area and higher temperature.Explanation:Powder has a larger surface area than chips, so powder reacts faster with dilute hydrochloric acid.A higher temperature increases the kinetic energy of particles, causing more frequent and energetic collisions.To get the slowest reaction, we need the smaller surface area (chips) and the lower temperature (20∘C).Answer:Option A: Calcium trioxocarbonate(IV) chips at 20∘C.