Exercise 3 solution

(a) Blurred background in a portrait: aperture priority; you pick a wide opening (low f-number, shallow depth of field, Chapter 3); the camera sets the shutter speed.
(b) Freezing a moving subject: shutter priority; you pick a fast speed; the camera sets the aperture.
(c) Quick snapshot: program mode; the camera sets both.
Manual is the alternative when you want to set both yourself, for example when lighting is awkward and you do not want the camera to change anything.