What is not appropriate to include in a letter of complaint?
details about the purchase price, model, and serial number
details about why you were dissatisfied with the product or service
a proposed solution to the issue
threats notifying the recipient of action you will take if not satisfied

The correct answer is D. Threats notifying the recipient of action you will take if not satisfied

Explanation:

A letter of complaint is a formal document used to communicate your dissatisfaction in relation to some product or service with the objective to ask for some solution. As this is a formal tool for communication it usually follow a specific format and should include information of the product or service you are complaining about, a summary of the reasons or situations that lead you to be dissatisfied, if you have taken any action by yourself to try to solve the problem and what you expect the company do for you, including any suggestion on how to solve the issue. Additionally,  any personal attack, rude words, threats or insults should be avoided as this is a formal document and also, can make the company be less willing to help you. Thus, it is never appropriate to include threats in a letter of complaint.

the writer needs to avoid using abbreviations and check spelling before sharing.