Microteaching becomes most useful while the trainee is practising a specific teaching skill.
Why B fits: A short practice lesson allows the trainee to apply a skill, receive focused feedback, and try an improved version.
Why the others are incomplete: Preparation sets the goal and later reflection helps improvement, but the teaching-practice stage provides the actual performance to observe and discuss.
Practical point: Microteaching usually focuses on a limited skill and a small teaching episode so feedback remains specific and manageable.