Option 1: hold back -> means to restrain or withhold oneself, which is the opposite of what a teacher would encourage.
Option 2: move on -> means to proceed to the next topic, which wouldn't help students who are confused.
Option 3: speak up -> means to express oneself or voice concerns, allowing students to ask questions when confused.
Option 4: step aside -> means to move out of the way, which is irrelevant in this learning context.
Hence, Option 3: speak up -> A good teacher wants students to vocalize their confusion and ask questions rather than staying silent. "Speak up" perfectly captures the act of voicing concerns or seeking clarification in a classroom setting. -> correct