Which sentence illustrates an appropriate tone and vocabulary for a letter of complaint? your product is completely worthless! the xyz broke the first time i used it. your new xyz isn’t worth the package it came in. xyz should be called why instead, because why would anyone want it?

I think that a letter of complaint should not be unnecessarily aggressive or insulting: after all, the people reading it are probably not responsible for the failure, but are just employed by the company.

I would recommend to stay away from the agrressive tone in:
Your product is completely worthless!
Your new XYZ isn’t worth the package it came in.

And of the passive aggressive tone in:
XYZ should be called WHY instead, because why would anyone want it?

So the best answer is:
he XYZ broke the first time I used it.