You are building a MobX store for a to-do application. Whenever a task is marked as completed, you want to automatically update the total count of completed tasks. Which MobX feature would be best suited for this?

In this scenario, a Computed Property in MobX would be best suited. Computed properties automatically update whenever the observable data they depend on changes. By defining a computed property for the total count of completed tasks, you ensure it updates automatically when the completion status of tasks changes. It's an efficient way to derive data from observables.