Exercise 3 solution

(a) Still landscape: single-shot (focus locks on confirmation and the subject does not move). Either detection method works; contrast detection's slowness does not matter for a static scene.
(b) Child running: continuous focus, which predicts the position of a moving subject from its speed and acceleration. Phase detection is preferable: it is fast and gives the direction to move the lens, whereas contrast detection gives no direction and is harder for moving subjects.
The choices follow the autofocus table; the practical advice is my own summary (check).