Which of the following is essential for effective teaching?

Communication skills are essential for effective teaching because they enable instructors to convey information clearly and engage students in the learning process. Effective communication involves not only presenting content in an understandable way but also actively listening to students' questions and concerns. This creates a supportive learning environment where students feel comfortable participating and expressing their thoughts.

While flexibility and strict discipline may play roles in teaching effectiveness, they are not foundational in the same manner as communication skills. Flexibility allows instructors to adapt their teaching methods to meet diverse learning styles and needs, but without strong communication, these adjustments may fall flat. Strict discipline might help in managing a classroom, but it does not contribute to the core educational objectives of understanding and engagement as effectively as communication does. Thus, while those aspects can complement teaching, communication is the fundamental skill that directly influences the effectiveness of instruction.