The bone, - broken in two places-, healed slowly. Which of the following best describes the underlined phrase? - essential present participial phrase-essential past participial phrase-nonessential past participial phrase-nonessential present participial phrase

This is best described as an essential past participle phrase.

It is essential, because the participle explains what happened to the bone to make it heal slowly. It is a past participle, because the bone is already broken, and has already healed (or started to heal).