What indicates that the teacher is frustrated?

The indication of the teacher's frustration is best represented by losing his temper. Losing one's temper typically reflects an emotional response characterized by anger or irritation, which aligns directly with feelings of frustration. When a teacher loses their temper, it suggests that they have reached a limit of patience or tolerance, often due to challenges in managing the class or difficulty with student behavior.

The other choices suggest different reactions that do not directly convey frustration. For example, assigning extra homework might be a method of reinforcing learning or addressing performance issues rather than an expression of personal frustration. Ignoring students can stem from a variety of reasons, such as being busy or unprepared, but it doesn't explicitly communicate the teacher's emotional state. Laughing at students typically does not indicate frustration; instead, it may reflect a humorous or light-hearted interaction. Thus, losing one's temper distinctly signifies the emotional struggle often associated with frustration.