Suggesting her with some points for her essay would be the appropriate action in this situation.

Explanation:

Identifying the Situation:
Your younger sister needs help with completing her school essay. As her elder sibling, it is important to guide and support her in her academic endeavors.

Understanding the Options:
a) Complaining to her teacher: Complaining to her teacher would not be a productive solution in this case. It is not necessary to involve the teacher unless there is a serious issue that requires their attention.

b) Complaining to the principal: Complaining to the principal would be an excessive response for a situation that can be resolved within the family. It may create unnecessary complications and strain relationships.

c) Suggesting her with some points for her essay: This option is the most suitable one. By suggesting her with some points for her essay, you are helping her develop her own ideas and encouraging her to think critically. This will not only improve her writing skills but also foster her independence and confidence.

d) Teasing her: Teasing your younger sister is not a constructive approach. It can demotivate her and create a negative learning environment. It is important to support and encourage her instead.

Justification for Option C:
Suggesting her with some points for her essay is the best choice as it allows you to help your sister without doing the work for her. By providing guidance and brainstorming ideas, you are empowering her to take ownership of her assignment, enhance her creativity, and develop her writing skills.

Conclusion:
Choosing option C, suggesting her with some points for her essay, is the most appropriate action in this situation. It promotes a positive learning environment and encourages your sister's growth and development.